All for Reef and High Magnesium

bakeman2022 · Jan 7, 2026

Has anyone ever had a situation where you have had very high magnesium while dosing All For Reef (AFR) and start having issues with their inverts dying?
Any input would be appreciated it.

Thank you.

Read on if you want more details on why I'm asking the question:
I have a 180 Gallon tank that has been up and running for almost 2 years now.
I thought about raising my Magnesium a little higher since I've heard that hammer head corals etc. like the higher value.
So I started to dose Red Sea Foundation C to get my Mag around mid 1400's and my hammer head started to flourish.

I then purchase a high end Mag/Cal/Alk salt mix (Fritz Red Box) and started using that for some water changes.
That is when I started to see the issue.
After my 2nd water change (about 30 gallons each water change), I start noticing issue with my snails - falling off rock, sluggish, and dead.
I read that high magnesium can cause issues with inverts. Tested my magnesium (which before the water changes was mid 1400's) and it was off the scale.

Immediately stop dosing Foundation C and did 2 more water changes over the next couple of weeks - which made my magnesium even higher (well over 1600).
Stopped using Fritz salt mix and purchase some low mag salt from red sea. Started to do water changes with that salt.
Mag came down to high 1400 but I was still seeing issues with my inverts.
During this time I lost 2 urchins in 48 hours from on-line store, purchase 2 more from LFS and lost them in less then 36 hours - so I knew I had a tank issue.
That is when I saw an AI results that says high magnesium and AFR can cause issues with inverts.

So I stopped dosing AFR. Waited about a week. Mag was hanging about 1450-1475 so I decided to purchased another urchin from LFS.
It was doing fine for the first few weeks. I was continually testing mag daily with 3 different mag test - always around high 1400s - however, I did have my LFS test my water a couple of times during this time and my mag from his tester was coming back at 1700 to 1750 ppm. So I'm not sure what to believe. I did do an ICP test a week before all this happened and everything look great (mag was around 1380) except my phosphate was a little high - my GFO media need to be changed.

In any case, since my urchin was doing fine I started my AFR dosing again.
Waited a couple of days and my urchin still seem to be doing OK so I purchased 2 more urchins from my LFS (I have GHA I'm trying to get rid of).
A day later one of my new urchin is dead and my other two urchin are acting sluggish.
So I stopped AFR again and now I'm going to start doing weekly water changes for the next month using instant ocean salt mix (which I used when I started the tank) to see if I can get this resolved.

Conclusion:
At first I thought it had to be the Fritz Red Box salt (which didn't make sense) but it wasn't until I stopped dosing AFR that my inverts stopped dying.
Not sure if its a combination of Fritz Red Box salt and AFR or just AFR and high Mag.
In any case, I'm stopping AFR for now, will dose Foundation B (calcium is always good - staying around 450 to 480) to keep ALK going while I do my weekly water changes.
Going to shoot for mag to be around 1300 to 1350 and see how things fair.

Water Parameters are good:
PH - swings between 8.4 and 8.3
Temp - swings between 77 and 78
Nitrate - 3 to 4 ppm
Phosphate (struggling a little bit with them - Using GFO) - .1 to .15
Mag - 1450 to 1475 (working to get that down through water changes over the next month)
Ammonia - 0
Nitrite - 0
Calcium - 450 to 480 ppm (Calcium will go up if my ALK goes down)
ALK - 9.3 to 9.7 dhk
Salinity - 1.025 (~35)

ICP test shows no metals in the water etc.
Iodine is a little low but not bad.
Also showed elevated levels of silicone but LFS told me everyone in our area have elevated levels of silicone in the water.
(But silicone in the water has always been there from day one.)
Recommendation from ICP was to check water source for silicone and change out GFO media for slightly elevated phosphate.

180 gallon tank with 40 gallon sump with refugium and protein skimmer.
2 MP 40s power heads as well as 2 separate return lines using 2 Syncra SDC 7/9 return pumps (so getting plenty of flow) and 3 Kessil A360x lights.

Corals seem to be doing fine (I have a elegant coral that is really flourishing).
One head on my frog spawn seems to bleach out a bit for a day or two but then recovers.
My toadstool has pulled in its polyps - at first I thought it was doing its molting thing but that was over a month ago. He just barely lets his polyps come out now.
I do have a rittera anemone that started to get a swollen month when this all started. His month is still swollen - Its not gapping - its definitely shut.
I did lose a peppermint anemone when this all started.
I have 20 fish that are doing fine - in fact my clown fish are still laying eggs.
I also have 2 clams that are doing fine.
I have lost a lot of my CUC. Still have blue legged hermit crabs, some P crabs, and a few emerald crabs but most of my snails (as well as all but 2 of my urchins) have died.
So I'm going to keep an eye on my Rittera and toadstool to see if they start making a recovery as I do my weekly water changes.

rtparty · Jan 7, 2026

Chasing numbers seems to be the main issue. There is no evidence that higher magnesium benefits LPS

https://www.reef2reef.com/threads/just-say-no-to-magnesium-testing-rmm-is-born.1017528/

bakeman2022 · Jan 7, 2026

rtparty said:
Chasing numbers seems to be the main issue. There is no evidence that higher magnesium benefits LPS

https://www.reef2reef.com/threads/just-say-no-to-magnesium-testing-rmm-is-born.1017528/

rtparty - ty for you input. I would agree that testing for mag is not ideal. I think the hanna test (which is one of them that I use) has a 10% tolerance - so given a value of say 1400 means it really could be between 1260 and 1540 - that's a huge delta.

Even though mag is what got me in this mess, it still begs the question what is killing my inverts. To me, it has to be a combination of high level of magnesium and dosing AFR together. The whole reason I started using AFR in the first place is because I do not want to do water changes. 20 gallons a week (10%) is a lot of water to change out. Plus you still have to do some A/B process to keep ALK/Cal in check (even with water changes). So AFR seemed to be a good solution.

So I'm hoping that others have had an issue with High Mag and AFR killing their inverts. If so, I'll just make sure I get my mag at a lower level (below 1400) and then use a low level mag salt (for those occasional water changes) and keep AFR dosing to keep alk, cal, mag, and trace elements in check.

rtparty · Jan 8, 2026

bakeman2022 said:
rtparty - ty for you input. I would agree that testing for mag is not ideal. I think the hanna test (which is one of them that I use) has a 10% tolerance - so given a value of say 1400 means it really could be between 1260 and 1540 - that's a huge delta.

Even though mag is what got me in this mess, it still begs the question what is killing my inverts. To me, it has to be a combination of high level of magnesium and dosing AFR together. The whole reason I started using AFR in the first place is because I do not want to do water changes. 20 gallons a week (10%) is a lot of water to change out. Plus you still have to do some A/B process to keep ALK/Cal in check (even with water changes). So AFR seemed to be a good solution.

So I'm hoping that others have had an issue with High Mag and AFR killing their inverts. If so, I'll just make sure I get my mag at a lower level (below 1400) and then use a low level mag salt (for those occasional water changes) and keep AFR dosing to keep alk, cal, mag, and trace elements in check.

There is no evidence of high magnesium killing inverts. Only anecdotal evidence that is unreliable at best.

High magnesium hurting inverts seems to come from the days of using Kent Tech M to kill bryopsis and some people claimed the higher the magnesium went, their snails moved less. Hardly scientific. It was later found that high magnesium is not what was killing bryopsis. Instead, it was an impurity specifically found in Kent Tech M. Even then it wasn’t a 100% success rate. So who really knows?

There are thousands of people using AFR and not killing inverts, me included. The snails in the tank I dose AFR to spawn like crazy. To the point I have to pull the babies out and put them in my larger tank every couple months or they starve in the small tank. To add enough magnesium with AFR to raise it so high it killed things, you’d spike alkalinity and calcium sky high and do far more damage

I will play devils advocate here and say, if dosing AFR you should be doing so BECAUSE you plan to change water. Water changes are the one thing that will keep AFR in check long term. Sure, there are reefers out there running AFR and doing no water changes. They are the exception and I would bet most are pretty advanced. I would pick something like the NYOS 2 part instead where I could dose alkalinity and calcium separately but also get the trace elements.

I don’t mean to bash Fritz but you would not be the first to use Fritz salt and see an immediate decline in animals.

Randy Holmes-Farley · Jan 8, 2026

I do not believe that AFR is a likely reason to have actual high magnesium. More people using it find it deficient in magnesium long term. Certainly, any tank where coralline algae is a significant part of the alk demand will find it deficient.

I recommend that folks add magnesium at 5-10% of the calcium dosed (the RMM method), and AFR is just below the low end of that recommendation.

exnisstech · Jan 8, 2026

I don't quite understand but I'm slow sometimes. You talk about dosing stuff for mag then want know if that along with AFR is killing your inverts?
I have no idea if the other stuff your mixing or high mag with AFR is killing your inverts but I'm confident AFR is not the cause.
There are many people using AFR with success and they have no problem with keeping inverts. I've been running ARF on a tank for over 2 years. I have no problem keeping inverts.
In addition to peppermint shrimp and a coral banded this Harlequin was in for 13 months before I moved it to another tank. The zebra turbo has been in for almost 3 years and is huge.

These limpets reproduce in the tank regularly.

Maybe high mag is a factor, I can't say because I have no idea where my mag is since i've never tested it. I can say with confidence that IME AFR isn't the cause of inverts dying.

bakeman2022 · Jan 9, 2026

rtparty said:
There is no evidence of high magnesium killing inverts. Only anecdotal evidence that is unreliable at best.

High magnesium hurting inverts seems to come from the days of using Kent Tech M to kill bryopsis and some people claimed the higher the magnesium went, their snails moved less. Hardly scientific. It was later found that high magnesium is not what was killing bryopsis. Instead, it was an impurity specifically found in Kent Tech M. Even then it wasn’t a 100% success rate. So who really knows?

There are thousands of people using AFR and not killing inverts, me included. The snails in the tank I dose AFR to spawn like crazy. To the point I have to pull the babies out and put them in my larger tank every couple months or they starve in the small tank. To add enough magnesium with AFR to raise it so high it killed things, you’d spike alkalinity and calcium sky high and do far more damage

I will play devils advocate here and say, if dosing AFR you should be doing so BECAUSE you plan to change water. Water changes are the one thing that will keep AFR in check long term. Sure, there are reefers out there running AFR and doing no water changes. They are the exception and I would bet most are pretty advanced. I would pick something like the NYOS 2 part instead where I could dose alkalinity and calcium separately but also get the trace elements.

I don’t mean to bash Fritz but you would not be the first to use Fritz salt and see an immediate decline in animals.

rtparty

Thanks again for your input. Actually, Fritz was the first place that I went when I started to lose inverts. In fact I gave 1/2 of a box back to my LFS because I didn't trust it. I've been using AFR since day one (almost 2 years ago) and everything was fine until I started to used Fritz. Plus, it seemed to me that things got better once I stop dosing AFR but that might have just been a fluke. I would like to continue using AFR if I can get trust again. I'll take a look at NYOS 2. Are you using NYOS2 and AFR on the same tank?

rtparty · Jan 9, 2026

bakeman2022 said:
rtparty

Thanks again for your input. Actually, Fritz was the first place that I went when I started to lose inverts. In fact I gave 1/2 of a box back to my LFS because I didn't trust it. I've been using AFR since day one (almost 2 years ago) and everything was fine until I started to used Fritz. Plus, it seemed to me that things got better once I stop dosing AFR but that might have just been a fluke. I would like to continue using AFR if I can get trust again. I'll take a look at NYOS 2. Are you using NYOS2 and AFR on the same tank?

Do you use NYOS 2 and if so do you use that with AFR as well?

I have 2 tanks: A 25g AIO that I dose with AFR and a 100g NYOS setup that I dose with the NYOS 2 part system.

The 25g only has a few corals and 1 fish in it. I bet most of my consumption in this tank is from coralline algae

bakeman2022 · Jan 9, 2026

exnisstech said:
I don't quite understand but I'm slow sometimes. You talk about dosing stuff for mag then want know if that along with AFR is killing your inverts?
I have no idea if the other stuff your mixing or high mag with AFR is killing your inverts but I'm confident AFR is not the cause.
There are many people using AFR with success and they have no problem with keeping inverts. I've been running ARF on a tank for over 2 years. I have no problem keeping inverts.
In addition to peppermint shrimp and a coral banded this Harlequin was in for 13 months before I moved it to another tank. The zebra turbo has been in for almost 3 years and is huge.

These limpets reproduce in the tank regularly.

Maybe high mag is a factor, I can't say because I have no idea where my mag is since i've never tested it. I can say with confidence that IME AFR isn't the cause of inverts dying.

Thank you for your reply.

Still trying to figure it out but you are correct. This all started when I was trying to raise my Mag to a higher value using Red Sea Foundation C. My initial attempt to do this was successfully but then I started to use Fritz Red Box (high in alk, cal, and mag) which caused my Mag to sky rocket and my inverts started to die. I immediately stop dosing mag, I stop using Fritz salt and I also stopped dosing AFR. Once I get my tank back stable again then I might try starting to use AFR again. Been using AFR since day one and haven't had any issues until I started using Fritz Red Box salt for water changes and dosing Red Sea Foundation C.