Oatmeal the Mantis Shrimp - RedSea Max Nano Peninsula Tank

[wwarby]

Background

I started in the hobby a little over two months ago with a Juwel Rio 240 litre (65 US gallons) tank and a female peacock mantis shrimp named Oatmeal (after the author of this comic). I put two small clownfish in there with her and she didn't treat them with the love and kindness I'd hoped for, instead chasing them around the tank with murder in her heart, so they were soon evacuated back to the LFS. I considered trying other fish but ultimately thought better of it - if Oatmeal didn't get on with them I'd end up having to sell them or give them away in a hurry. Leaving a tank this size with Oatmeal as the only inhabitant wasn't on the table so I decided to buy her a new home, and thought this time I'd start a tank thread from the outset to document my progress. My Juwel tank will remain where it is, and be repurposed as a mixed reef tank in due course.

My first tank build was a catalogue of errors and I was lucky that Oatmeal survived the new tank syndrome I put her through after a failed quick cycle with bottled bacteria. I've learned a lot over the past few weeks and this time I'm taking things a bit slower, making one decision at a time and hopefully will make fewer mistakes.

Tank Setup

The equipment I've bought and installed so far is:

• RedSea Max Peninsula 100 litre (26 US gallons) tank
• Stock ReefLED 50 light
• Stock skimmer
• H2Ocean compact ATO with 10 litre bucket for reservoir
• DD 150W titanium heater controlled by InkBird ITC-306A
• DD jump guard
• Flipper Float Medium magnetic cleaner
• Manual backup thermometer
• Meross wifi power strip (6 gang)

The tank is going to be situated on top of a kitchen counter with the plumbing and electrical running through a hollow wall into a nearby cabinet. I've already learned one important lesson during installation: don't turn on the heater when there's no water in the tank I've cleaned off the melted plastic using white vinegar.




Aquascape

The next decision I need to make is how best to disguise this enormous PVC tubing for Oatmeal's burrow. I'm building 3" (82mm) diameter U-shaped burrow per recommendation from @nmotz. The thinking is that the mantis needs to be able to jack-knife inside the burrow whilst molting, which she couldn't do in the 2" tubing I used in her first tank and she was visibly (and audibly) unhappy about it when she molted. I've looked at various possible configurations, grateful for thoughts on whether any of these are unacceptable. Whilst obviously prioritising the needs of the mantis shrimp first, I'm want to be able to clearly see both entrances (the left and bottom sides of the tank will be the most visible), and I want the tube to be as well hidden as possible under sand and rock. I intend to cover most of it with sand. and I was thinking of roughing the edges and gluing small rock fragments all around the top half outside with reef cement so that even if Oatmeal starts shovelling sand she won't expose bare black plastic - then adding my aquascape rocks on top of that base.

1	2	3	4	5	6	7

For sand I'm thinking of going with a mix of CaribSea Fiji Pink and a 5mm coral sand so that she has a good variety of different grain sizes for building with, and I intend to build the aquascape with around 10-20lbs of live rock. The sand bed will then be littered with a generous assortment of rock rubble which I'll move from my current tank.

Other Livestock

In all likelihood Oatmeal will be the only inhabitant, although I'm not ruling out the possibility of trying a faster and/or more aggressive fish as a tank mate when I have another tank that I can safely evacuate it to at the first sign of danger. I'll probably try my luck with cleanup crew at some point too when I have the opportunity to pick up some snails or hermits etc. on the cheap, expecting that she'll probably eat them at some point.

I would however definitely like to fill this tank with soft corals (strictly beginner species) and make the tank look pretty.

Oatmeal Herself





Next Steps

1. Settle on a burrow layout
2. Aquascape with sand and live rock
3. Cycle the tank - properly this time, for as long as it takes
4. Catch Oatmeal in a trap, and move her to her new home
5. Start buying fish for Oatmeal's former palace

 

[exnisstech]

That shrimp is too cool. Glad you decided to keep her.

 

[Mhamilton0911]

I love that you decided to keep her and try a smaller tank just her. A mixed reef with fish for the bigger tank you'll just love it. Saltwater fish have such personalities and are so unique I'm sure you'll find lots of cool inhabitants!!

 

[52728299]

Background

I started in the hobby a little over two months ago with a Juwel Rio 240 litre (65 US gallons) tank and a female peacock mantis shrimp named Oatmeal (after the author of this comic). I put two small clownfish in there with her and she didn't treat them with the love and kindness I'd hoped for, instead chasing them around the tank with murder in her heart, so they were soon evacuated back to the LFS. I considered trying other fish but ultimately thought better of it - if Oatmeal didn't get on with them I'd end up having to sell them or give them away in a hurry. Leaving a tank this size with Oatmeal as the only inhabitant wasn't on the table so I decided to buy her a new home, and thought this time I'd start a tank thread from the outset to document my progress. My Juwel tank will remain where it is, and be repurposed as a mixed reef tank in due course.

My first tank build was a catalogue of errors and I was lucky that Oatmeal survived the new tank syndrome I put her through after a failed quick cycle with bottled bacteria. I've learned a lot over the past few weeks and this time I'm taking things a bit slower, making one decision at a time and hopefully will make fewer mistakes.

Tank Setup

The equipment I've bought and installed so far is:

• RedSea Max Peninsula 100 litre (26 US gallons) tank
• Stock ReefLED 50 light
• Stock skimmer
• H2Ocean compact ATO with 10 litre bucket for reservoir
• DD 150W titanium heater controlled by InkBird ITC-306A
• DD jump guard
• Flipper Float Medium magnetic cleaner
• Manual backup thermometer
• Meross wifi power strip (6 gang)

The tank is going to be situated on top of a kitchen counter with the plumbing and electrical running through a hollow wall into a nearby cabinet. I've already learned one important lesson during installation: don't turn on the heater when there's no water in the tank I've cleaned off the melted plastic using white vinegar.




Aquascape

The next decision I need to make is how best to disguise this enormous PVC tubing for Oatmeal's burrow. I'm building 3" (82mm) diameter U-shaped burrow per recommendation from @nmotz. The thinking is that the mantis needs to be able to jack-knife inside the burrow whilst molting, which she couldn't do in the 2" tubing I used in her first tank and she was visibly (and audibly) unhappy about it when she molted. I've looked at various possible configurations, grateful for thoughts on whether any of these are unacceptable. Whilst obviously prioritising the needs of the mantis shrimp first, I'm want to be able to clearly see both entrances (the left and bottom sides of the tank will be the most visible), and I want the tube to be as well hidden as possible under sand and rock. I intend to cover most of it with sand. and I was thinking of roughing the edges and gluing small rock fragments all around the top half outside with reef cement so that even if Oatmeal starts shovelling sand she won't expose bare black plastic - then adding my aquascape rocks on top of that base.

1	2	3	4	5	6	7

For sand I'm thinking of going with a mix of CaribSea Fiji Pink and a 5mm coral sand so that she has a good variety of different grain sizes for building with, and I intend to build the aquascape with around 10-20lbs of live rock. The sand bed will then be littered with a generous assortment of rock rubble which I'll move from my current tank.

Other Livestock

In all likelihood Oatmeal will be the only inhabitant, although I'm not ruling out the possibility of trying a faster and/or more aggressive fish as a tank mate when I have another tank that I can safely evacuate it to at the first sign of danger. I'll probably try my luck with cleanup crew at some point too when I have the opportunity to pick up some snails or hermits etc. on the cheap, expecting that she'll probably eat them at some point.

I would however definitely like to fill this tank with soft corals (strictly beginner species) and make the tank look pretty.

Oatmeal Herself





Next Steps

1. Settle on a burrow layout
2. Aquascape with sand and live rock
3. Cycle the tank - properly this time, for as long as it takes
4. Catch Oatmeal in a trap, and move her to her new home
5. Start buying fish for Oatmeal's former palace

Excited to see how the mixed reef turns out
Imo a qt tank would be beneficial for new fish, putting them through the qt protocol.

 

[wwarby]

Excited to see how the mixed reef turns out
Imo a qt tank would be beneficial for new fish, putting them through the qt protocol.

Thanks @Aluco, yeah I'm definitely going to look into a quarantine tank for the mixed reef tank (and also for the corals that will hopefully be going into both tanks). Like probably every new reefer my first reaction to the idea was that I don't want some ugly bare bones tank in my house or the delay and procedure that comes with using it, but I've read enough and seen enough now to know that people do it for very sounds reasons that shouldn't be ignored. I'm gonna talk to my LFS about it this week.

 

[nmotz]

Great update, wonderful to see how you are learning. Per your graphics, burrow configuration 3 or 4 is best. You want a long (preferably 2 or more body lengths) straight section of pipe for them to feel completely hidden. They are more active when they know they have a dark comfortable place to just rest every now and then.

 

[wwarby]

Great update, wonderful to see how you are learning. Per your graphics, burrow configuration 3 or 4 is best. You want a long (preferably 2 or more body lengths) straight section of pipe for them to feel completely hidden. They are more active when they know they have a dark comfortable place to just rest every now and then.

I knew I should have waited for you before picking a burrow layout

This is the layout I chose, and I've been cementing pieces of rock to it since last night


The way I've done it, the top joint in the picture would be pretty dark but there's only about 4 inches of it's that's truly straight.

To get the straight section you're suggestion I'd need to break off all the rocks and start again. I'm never going to be able to make two full body lengths straight but I can add about 4 inches on the long edge and swap the 90 degree and 45 degree bends so that there's a section on the right that's as long and dark as is theoretically possible within the tank dimensions.

That layout will be less ideal for viewing angles because both entrances will face the same side of the tank and the end of the peninsula will be more of a "profile" view when she's poking her head out - which is fine, but just not quite as optimal for me the viewer. If that's what you think I should do, I'll do it - but before I rip up hours worth of rock gluing, do you definitely think this is important enough worth changing?

Incidentally what I was planning is to lay the pipework virtually flat on the bottom of the tank (maybe raised half an inch on some rock rubble at the openings) and fill with sand to at least an inch and a half at the front, and slightly more at the back. As such the sand level will be above the bottom of burrow entrance. I figure Oatmeal will kick out any sand that falls into the burrow if she doesn't want it here, but that by having the burrow low to the ground I (a) don't raise the rock any higher than necessary towards the water line (leaving more room for coral growth) and (b) make it easier for Oatmeal to close the burrow during molts because the sand be a stable starting point to build the drawbridge on top of. I'm planning to then add a couple of decent size pieces of live rock on top of the rock I've attached to the burrow, for aquascaping and eventual coral mounting. Any issues with any of that?

 

[nmotz]

This is a good burrow plan, don’t worry about it too much… the main thing is to minimize light pollution and give them room to turn/molt. If the burrow is 3” diameter you are good to go. The benefit of having straight sections is more for their comfort. Your epoxy work and rock attachments look well done. I think you’ll enjoy this setup a great deal.

 

[wwarby]

Awesome, thanks @nmotz. I was looking at the painstakingly arranged rock work today with dread at the thought of taking a crowbar to it so that’s a bit of a relief! Hopefully will get the tank wet at the weekend then and start cycling.

Oatmeal has been swimming around her tank very actively today - hope that continues!

 

[wwarby]

I've got water in the new tank now, all looking good so far. I used PVC tubing from Kockney Koi (sold by Pond Planet) which has an outer diameter of 82mm and inner diameter of 76mm (3"). Two 90 degree bends and one 45 degree bend held together with short sections of straight pipe. I scratched all over the surface of the pipe with a metal rasp to make it easier to stick things to it. Using about 10lbs of dry rock which I broke into pieces using a coal chisel, I covered the top half of the burrow in rock, held to the burrow with reef cement and putty.

The burrow sits flat on the tank floor, surrounded by about 20lbs of Caribsea Ocean Direct sand which contained various shells, coral fragments and rocks amongst the fine grain sand, covering an average depth of around 2". On top of the dry rock burrow are about 8lbs of live rock from my LFS in three chunks, held in place with putty. I've left a small dry rock island to eventually use for some kind of coral, and added quite a few pieces of rubble - a mix of live rock and dry rock fragments.





The tank has been running for two days now and everything seems to be working with a couple of caveats:

1. I'm struggling to work out the temperature of the water because literally every thermometer I own seems to disagree with every other one. I have no less than four mercury thermometers from three different brands and they span a range of 23.5 - 27.5 Celsius (74.3 - 81.5 Fahrenheit). I have a digital salinity meter with a thermometer built in that measures somewhere inside that range but I don't trust it. The one I trust the most is the Inkbird which reads 25.5 Celsius - just about where I want it to be. Who knew mercury thermometers could be so hopelessly unreliable?

2. The stock skimmer is too loud. Long term I'm not going to be able to live with that noise, so it either needs to get quieter or get turned off (or at least turned off during the day). I've heard they settle down over time so I'll leave it running for a while and see how I get on.

Tank parameters this morning:

Ammonia: 0
Nitrite: 0
Nitrate: 5ppm
Salinity: 1.025

The LFS did say that with the live rock and live sand the tank would cycle quickly, but it is possible the tank has already cycled? With my first tank it took a couple of weeks for my water quality to go downhill, so don't want to rush to move Oatmeal. Should I put something in the tank like a prawn or a cockle to generate ammonia and force the bacteria to do some work? or just leave it a week or two and keep testing?

 

[wwarby]

Thermometer issues now sorted, and the skimmer has calmed right down. I hear a bit of a "whirring" but no pump vibration noise now, and the trickling water sound is much quieter now that I've increased the water level in the last chamber. I've ordered the 3D printed baffle and filter media cup from eBay so I'll fit those anyway, but I'm happy with the noise level at this point.

The new tank has been cycling for a week with a decent amount of live rock in there that only spent about 30 minutes of water, so probably not much die off. Salinity dropped to 1.023 (probably due to my adjusting water levels with the ATO) so I've pulled that back up to 1.025 by mixing a bit of new salt into a bucket of tank water. Ammonia has stayed at 0, nitrite spiked a tiny bit to 0.025ppm but is back down to zero, nitrates are at 5ppm. Temperature is stable, at around 25.5 degrees Celsius (same as Oatmeal's current tank) and diatoms are starting to appear on the rock.

Is it time to move Oatmeal yet? I'm trying not to rush, but not sure is there's any reason to wait at this point. Nitrates are higher in Oatmeal's current tank than the new one at the moment (I think I may have caused a small spike to 15ppm by churning too much of the gravel in one go with my gravel vac. It's on it's way back down though).

 

[parclife]

I've got water in the new tank now, all looking good so far. I used PVC tubing from Kockney Koi (sold by Pond Planet) which has an outer diameter of 82mm and inner diameter of 76mm (3"). Two 90 degree bends and one 45 degree bend held together with short sections of straight pipe. I scratched all over the surface of the pipe with a metal rasp to make it easier to stick things to it. Using about 10lbs of dry rock which I broke into pieces using a coal chisel, I covered the top half of the burrow in rock, held to the burrow with reef cement and putty.

The burrow sits flat on the tank floor, surrounded by about 20lbs of Caribsea Ocean Direct sand which contained various shells, coral fragments and rocks amongst the fine grain sand, covering an average depth of around 2". On top of the dry rock burrow are about 8lbs of live rock from my LFS in three chunks, held in place with putty. I've left a small dry rock island to eventually use for some kind of coral, and added quite a few pieces of rubble - a mix of live rock and dry rock fragments.





The tank has been running for two days now and everything seems to be working with a couple of caveats:

1. I'm struggling to work out the temperature of the water because literally every thermometer I own seems to disagree with every other one. I have no less than four mercury thermometers from three different brands and they span a range of 23.5 - 27.5 Celsius (74.3 - 81.5 Fahrenheit). I have a digital salinity meter with a thermometer built in that measures somewhere inside that range but I don't trust it. The one I trust the most is the Inkbird which reads 25.5 Celsius - just about where I want it to be. Who knew mercury thermometers could be so hopelessly unreliable?

2. The stock skimmer is too loud. Long term I'm not going to be able to live with that noise, so it either needs to get quieter or get turned off (or at least turned off during the day). I've heard they settle down over time so I'll leave it running for a while and see how I get on.

Tank parameters this morning:

Ammonia: 0
Nitrite: 0
Nitrate: 5ppm
Salinity: 1.025

The LFS did say that with the live rock and live sand the tank would cycle quickly, but it is possible the tank has already cycled? With my first tank it took a couple of weeks for my water quality to go downhill, so don't want to rush to move Oatmeal. Should I put something in the tank like a prawn or a cockle to generate ammonia and force the bacteria to do some work? or just leave it a week or two and keep testing?

This set up looks amazing! Can’t wait to see her in there

 

[JoJosReef]

I love The Oatmeal! Great comic and great inspiration. Looks like this second tank is starting on the right foot.

Do mantis shrimps bother corals? How about a clam, would that become dinner? Lettuce nudibranch? Tux urchin? I should read up more on what is mantis compatible. Can't wait to see how this tank progresses!

 

[wwarby]

I love The Oatmeal! Great comic and great inspiration. Looks like this second tank is starting on the right foot.

Do mantis shrimps bother corals? How about a clam, would that become dinner? Lettuce nudibranch? Tux urchin? I should read up more on what is mantis compatible. Can't wait to see how this tank progresses!

I can answer all those questions in hypotheticals because I haven't actually tried any of them, but I do know the theoretical answers

Mantis shrimps don't intentionally bother corals. The biggest risk is they knock over frags that aren't well secured to the rock, because they did tend to crawl around over the rocks and they're pretty chunky (mine is about the size and shape of a banana). Frags attached to small rocks on the sand are also vulnerable to being rearranged or tipped upside down because the mantis likes to move pieces of loose rock rubble around - especially when molting because they use them to barricade the burrow entrances.

Clams would almost certainly become dinner - they'll eat anything meaty but the smasher mantis shrimps have evolved to break shells. Mine loves cockles and I'm told that half clam on the shell is one of the best foods you can get for them (I've actually ordered some of those - they're not so easy to get hold of in the UK).

Nudibranch shouldn't become dinner, and I know somebody who kept nudibranch in his mantis tank to help keep aiptasia under control - it's something I might try if I have an aip outbreak.

Urchins are not a natural food for a mantis shrimp as far as I'm aware but they might run into trouble if they stumble into the burrow. There's a general rule with a mantis tank that you don't put anything in it you're not willing to lose. Some have had success with fish, others less so. One guy I know has a yellow tang that follows his mantis around like best buds, others have had success with damsels, chromis, royal gramma, six line wrasse and mollies. I know of two reefers who have successfully kept clowns with their mantis - when I tried that though she terrorised them. I'm leaning back towards trying fish in her tank again at some point since I now have the means to immediately move them to a safer environment if they're threatened, but I'm not in any hurry to try that any time soon.

 

[JoJosReef]

Good to know that berghia can make it in the tank and clean up aiptasia. Doubt peppermints would fare well. A few lettuce nudibranches are interesting visual additions if/when algae starts to make an appearance, and they would be a part of the CUC that snails might not be able to fill--moderate expectations, though. Chitons maybe?

Do you know if display macros like Brotycladia or dragons breath would help fish in terms of hidey spots? I'm assuming a good sized anemone wouldn't keep a mantis at bay and provide safe haven for clowns--although I can't imagine a better scenario to get you clowns to accept an anemone as host!

 

[wwarby]

Yeah peppermints would definitely be on the menu. When I put river shrimp in the tank, Oatmeal immediately goes into incredible hulk mode and hunts them down one by one - they don't last long at all. I've just this evening put a tuxedo urchin in with her, along with a turbo snail, a trochus snail and a money cowrie. The tuxedo snail I have high hopes for - I've not been able to find any accounts of urchins being predated by mantis shrimps but obviously anything that goes in the tank is at some level of risk. The snails I expect to lose, but I tried one of each to see if she leaves any of them alone. Some people have success with snails and hermits so I think it's worth experimenting.

I know @nmotz keeps macro algae with his mantis to help keep nitrates down rather than for hidey spots. There's another mantis owner here who keeps clowns protected by a carpet anemone. What I'm coming to understand about mantis shrimps is that it's hit and miss whether they see something as food or not. Perhaps may have hunted fish in the wild and got a taste for it. I've heard of them carrying hermit crabs out of their burrows and living for months with decorator crabs, emerald crabs and various species of fish. Mine never actually caught my clowns but lunged toward them several times before I couldn't stand to watch it any more and evacuated them. The general rule is don't put anything in there you're not willing to lose, but that doesn't mean you're guaranteed to lose anything.

 

[wwarby]

Well, after testing the water parameters again this morning in Oatmeal's tank and finding they were still looking good, I decided it was time to make the move today. Catching her went exactly to plan - I put half a prawn in a homemade trap comprising a 2 litre water bottle with the top cut off and inverted, and the tip enlarged just enough for a mantis shrimp to fit through. I also put a bit of rubble rock in there to weigh it down and placed it in the tank. She was inside it within 60 seconds (she'd already eaten a massive prawn just last night, but she's a glutton) so I immediately covered the open end with a net, lifted the trap out and gently released her into a bucket of her own tank water after removing the cut off bottle top.

I drip acclimated the bucket for about two hours to reach about 3x the original volume of water. I wondered if I ought to have dripped slightly faster and removed some of the water part way though the process but I figured that might have stressed her out and after a couple of hours I was getting slightly concerned about temperature drop so figured it was best to make the move. After about 10 minutes of recovering from the stress of the move hiding behind the rockwork, she found her new burrow and signalled her approval by moving in. She's been poking her head out a little and already started reorganising the rock rubble so I think she's taken to the move well. I've given her loads of rock rubble in varying sizes (mostly live rock fragments) so she has plenty of options for building materials.

Yesterday I added my first two coral frags to the tank, along with a tuxedo urchin. The urchin may end up on the menu, but I couldn't find much evidence of mantis shrimps attacking urchins and I figured it was worth a punt - if she leaves it alone then it'll hopefully get to work on the algae that is already starting to cover the rocks. The frags are nothing fancy - GSP and some kind of fuzzy green mushroom. I've got a handful of zoas arriving on Thursday and couple of other softies - for now I only intend to keep the very easiest beginner species of corals in this tank.

Meanwhile Oatmeal's former home is now cleared of ugly plastic tubing, discarded cockle shell fragments and rock rubble, and is looking a bit more spacious as I've been able to move the aquascape backwards a bit now that it doesn't have to sit in front of a 2" plastic tube. Clowns are in QT now, waiting to become the first inhabitants of the newly vacant tank. I'll start a separate tank thread to document my progress on that tank's reincarnation as a community reef

 

[Stomatopods17]

Is there a way to attach a fuge or increase the water volume at all with that setup?

Shell rot kills and its a very common (basically guaranteed in a regular setup) issue with O. scyllarus. Don't know what exactly it is (some say its lighting but they'd go extinct in Indonesia if that was the case), odds are like algae it just blooms randomly at the sudden spike of nitrates and once its there its *very* difficult to cure (basically depends on location and how the molts go, your control is limited to nature).

I wouldn't have downsized from the old build unless i could grow mangrove roots to outcompete shell rot or get some kind of skimmer going. Setup is still fresh so hopefully it can be caught before it happens. O. Scyllarus is hardy enough to handle a lot of abuse with tank quality but the silent slow killer is the shell rot that develops which turns it from 100 to 0 on the fragility scale.

 

[wwarby]

Is there a way to attach a fuge or increase the water volume at all with that setup?

Shell rot kills and its a very common (basically guaranteed in a regular setup) issue with O. scyllarus. Don't know what exactly it is (some say its lighting but they'd go extinct in Indonesia if that was the case), odds are like algae it just blooms randomly at the sudden spike of nitrates and once its there its *very* difficult to cure (basically depends on location and how the molts go, your control is limited to nature).

I wouldn't have downsized from the old build unless i could grow mangrove roots to outcompete shell rot or get some kind of skimmer going. Setup is still fresh so hopefully it can be caught before it happens. O. Scyllarus is hardy enough to handle a lot of abuse with tank quality but the silent slow killer is the shell rot that develops which turns it from 100 to 0 on the fragility scale.

I definitely don’t have any practical way of increasing the water volume unfortunately - there’s just no space for it where I’ve put the tank. I do keep a very close eye out for shell rot and I can’t see any sign of it at the moment. As you say, it doesn’t seem to be entirely clear what causes it, but I try not to overdo it with the lighting and I’m monitoring nitrates closely.

I’m committed to this setup now really - the larger tank just felt empty and I wasn’t enjoying it. I sought opinions on the new tank size and got several - consensus seemed to be that 100 litres is fine, and so far Oatmeal does indeed seem to be settling in really well.

She’s more active than she used to be and guards her burrow entrance 80% of the time now so I can see her whenever I want. She frequently leaves the burrow to explore, and she’s constantly rearranging the rubble around her burrow entrance.

The tank looks ugly at the moment thanks to an aggressive green hair algae bloom, but she’s killed two snails and a tuxedo urchin since I moved her in so she’s not getting any more CUC to help with the gardening!

 

[Stomatopods17]

Hair algae hopefully would out compete it but if its growing there's def nitrates feeding it.

Lighting is more or less just a correlation, your setup is fine but most keep them with inproper burrows so they're more 'exposed and stressed' without the dark areas to feel secure in. I don't think keeping one in a reef tank vs. no lights are all would be any different if the water quality is perfect. Lighting implies shell rot is photosynthetic which is possible but we don't really know for sure. Big mantis shrimp are messy so nitrate spikes are a given and shell rot development can be as small as a tiny spec that either grows or scars.

Get some macro algae that's pretty to look at, like dragon's breath, mangrove, etc, mantis shouldn't bother them and they'll drain nutrients out of the water for hair algae, starving it, would substitute for a CUC that way.

EDIT: I just watched one of those videos to see if I could spot anything, this is the spot you need to watch for, this is def shell rot on the carapace above the right arm, check if its still there after the molt or if it scar'd:

about 0:07 you can see it in the 26second video.