2 Gallon (7.5L) Cookie Jar Reef

[pietervanwhelp]

Hey everyone,

To start, I am a total beginner. I saw someone building a pico reef jar on Instagram and thought it looked incredible — minimal, bizarre, a little risky, and fascinating. I've never had any saltwater tanks before (only a few easy freshwater ones), but after some more inspiration and research on the internet, I am (maybe foolishly) going for it. I know smaller tanks are harder to manage and can swing quicker, but I don't have a big budget or a lot of space, and was just really intrigued by the whole look and feel of it. Who knows if this will be a long-term hobby that evolves larger and larger like I'm told it will, or just a short-term fun experiment. Excited to find out.

I'm based in Amsterdam, the Netherlands. My local reef shop (Aquaasan Corals, near Schiphol) let me hand-pick live rock, and also supplied live sand, pre-mixed saltwater, and RO water for top-offs. I came home with everything I needed and a mild sense of panic.

The build​

• Vessel: 7.5L cookie jar, 24cm wide
• Light: Hygger HG088 30W clip-on, marine/saltwater version
• Flow: Nicrew Nano Air Pump 18L/H — no wavemaker at this volume, just surface agitation
• Heater: Hygger Mini Pro HG031 10W — self-contained, built-in display
• Water: Pre-mixed saltwater from Aquaasan; RO water from the same store for top-offs

Day 1​

Sand went in first, then the live rock. I didn't rinse either of them. I went for one chunky piece with visible coralline (purple and pink — apparently a good sign) and another slightly larger second piece. Then in went the saltwater, which immediately turned everything a milky white cloud. I used the "bubble wrap" method to add the water but didn't do the best job.

Turns out this is pretty normal — sand particles settling and the bacterial bloom kicking off. Still unsettling when you're staring at what looks like a snow globe on a build with no filter. Potential noobie mistake #1, I ordered the light/heater/air pump to arrive on the same day as I was getting the rock/sand. But it was delayed and didn't arrive until the next day. I also left the lid on, which seals it pretty well. Therefore, after a few hours when I got my first water test results, the pH was a bit low (7.6), my hardness was high, chlorine wasn't at 0, and nitrates/nitrites were at 0. I retested to be sure with same results. Its possible CO2 was building up and couldn't release since I left the lid on and didn't have my pump agitating the surface. They all arrive tomorrow so I'll set them up and give you guys an update on the build and water tests.

Questions for the community​

1. Do I have too little sand and too much rock? Its around and inch or two of sand and I have one larger rock and one small one — it doesn't look packed but I have no reference point.
2. Best placement for the air pump tube and heater inside a round jar? Currently just guessing.
3. Water change frequency during cycling — how often, how much?
4. First soft coral recommendation once things stabilise?

The plan

I'm going soft corals and LPS only — zoanthids, mushrooms, maybe a Duncan once things stabilize. Clean-up crew first (small snails, maybe a sexy shrimp or peppermint), then corals in phases. Fish undecided — probably keeping it invertebrate-only.

Hope some of you can follow along and help me out!

 

[Fish Fan]

Nice looking pico build; following!

 

[smitten with ocean life]

@Calmwaters would be able to give you some tips

 

[Calmwaters]

My biggest tip is just make sure your equipment is running every day and make sure you keep an eye on salinity because in a tank that small just a little evaporation can cause major swings. Good luck with it I look forward to watching your progress.

 

[pietervanwhelp]

My biggest tip is just make sure your equipment is running every day and make sure you keep an eye on salinity because in a tank that small just a little evaporation can cause major swings. Good luck with it I look forward to watching your progress.

Thanks! I've got it sitting in my living room, so definitely having a look each day and always looking for improvements. I'm only on day 3 but I can feel the obsession growing

 

[Calmwaters]

Thanks! I've got it sitting in my living room, so definitely having a look each day and always looking for improvements. I'm only on day 3 but I can feel the obsession growing

Aquariums are addictive for sure! When you decide to go bigger or get more keep an eye out for used tanks/equipment that will save you a bunch of money. Lol

 

[Gumbies R Us]

Impressive! I’ll be following this to see how this turns out!

 

[pietervanwhelp]

Update on Day 2:

We are in business! Heater is set up at 25C (78F), air pump is bubbling, and the light is set up clamped to the side of the jar. Looks great in my opinion

The milky cloud from day 1 is completely gone. You can see the two pieces of rock clearly now — one larger flat piece on the left, one chunkier piece on the right with some green coralline starting to show. Sand bed looks clean. There's a decent open area on the sand which I'm hoping will be good for future clean up crew movement.

pH scare (resolved)​

Day 1 test showed pH at ~7.6, which sent me into a minor spiral. Traced it to CO₂ buildup — the lid was on and the air pump was still in the mail, so there was no surface agitation. Once the pump arrived and I got some movement at the surface, pH climbed back to 8.0 by day 2. Targeting 8.1–8.3. Lesson learned the hard way: surface agitation matters a lot in a sealed jar.

Parameters so far​

• Day 1: pH 7.6 · KH 10 · Nitrate 0 · Nitrite 0
• Day 2: pH 8.0 · KH 10-15 · Nitrate 0 · Nitrite 0

One thing to note is that I keep getting a chlorine reading at around 0.8 when I'm pretty sure its supposed to be at 0. My AI assistant says its likely a false positive from saltwater interference? Should it be a bigger concern of mine? Also, my testing strips are not the highest quality so I take it all with a grain of salt.

Also my salinity refractometer arrived today. I calibrated it at 0 with some RO/DI water and then took a reading, it looks to be right at 1.025, maybe a tiny bit below.

I ordered a separate kit for Ammonia testing, but I don't think it should have spiked this early, we'll see soon.

Still looking for advice on​

1. Rock load — does two pieces look right for 7.5L? (Top-down photo above gives a good view of the layout.)
2. Pump and heater placement — the air line is running down the back center, heater is tucked to the right behind the rock. Any suggestions for better positioning?
3. Light height -- higher/lower? I have it on 8h per day at this point
4. I don't know exactly what to do with the lid of the jar, its currently resting on top due to the light/heater/pump cords as you can see in the third photo. Ok to leave like that or should it be more secure/sealed?
5. Water changes during cycling

The journey continues. Watching, waiting, testing daily

 

[pietervanwhelp]

Cookie jar week 1 update — ammonia just showed up and I'm a little excited about it. Something is actually happening in there.

Some hitchhikers or something else?​

The API kit arrived​

Took the advice and got an API Saltwater Master Test Kit — glad I did, because it immediately showed something my old kit couldn't: ammonia at 0.50 ppm. The cycle has officially started. I know that means nothing can go in the tank for a while, but seeing that number actually felt like progress rather than bad news.

Thanks to everyone who flagged the salinity. Measuring daily now with the refractometer and sitting at 1.024–1.025 which seems stable so far. Topping off with RO water as needed but I'm getting very little evaporation so only done it once or twice.

Week 1 parameters​

Parameter	Reading	Status
pH	8.2–8.4	Good (I think)
Ammonia	0.50 ppm	Spike beginning — expected
Nitrite	0	Not yet — coming soon
Nitrate	0	Nothing
Salinity	1.024–1.025	On target
KH	10 dKH	Stable
Hardness	>21	Fine?
Chlorine	~0.8 (dropping)	Likely false positive in saltwater

The chlorine reading is something I'm not stressing about — from what I've read, standard chlorine tests give false positives in saltwater. If anyone knows differently, happy to be corrected.

What I'm watching for next​

From what I understand, the sequence from here is: ammonia climbs, then nitrite spikes as bacteria start processing it, then nitrate begins to accumulate, and ammonia/nitrite drop back to zero. When that happens the tank is cycled. I'm testing every day and logging everything.

For anyone following along — thanks for the advice so far, it's genuinely been helpful. How long did your first cycle take with live rock? Curious whether I should expect 2 weeks or closer to 4.

Something I wonder about here, I get a little build up of bubbles around the cords/light bracket. I was thinking about getting a different lid that fits a little better around the wires, since for now it is kind of precariously propped up on them, and getting a stand that I could clamp the light to instead of having it on the side of the jar. Not sure yet.

 

[PolypPocket]

Looks great! I saw that video too…the cookie jar guy

 

[kizzy89]

I have a reef jar, i have ula opae shrimp (Hawaiian volcanic shrimp) aclimated to reef conditions. Something to look into if you want lots of movement in a jar. Yours looks good also!

 

[pietervanwhelp]

I have a reef jar, i have ula opae shrimp (Hawaiian volcanic shrimp) aclimated to reef conditions. Something to look into if you want lots of movement in a jar. Yours looks good also!

@kizzy89 I would love to get a few shrimp here in a couple weeks, do you have any pictures or posts about your setup?

 

[kizzy89]

I have a reef jar, i have ula opae shrimp (Hawaiian volcanic shrimp) aclimated to reef conditions. Something to look into if you want lots of movement in a jar. Yours looks good also!

@kizzy89 I would love to get a few shrimp here in a couple weeks, do you have any pictures or posts about your setup?

Unfortunately i had flatworms on a coral and they're all over it at the moment plus a little cyno, I'm too ashamed to post a photo of it lol it's hard to get decent flow in a jar, i may transfer the shrimp over to a 3 gallon cube after i get one set up but they don't seem to mind the current setup at the moment. What are you going to do for flow in your jar?

Edit: i seen you're only going to have a air pump, that's what i did and it didn't work out so great, maybe you have a better way to do it but i couldn't come up with anything and it backfired slightly.

 

[pietervanwhelp]

Unfortunately i had flatworms on a coral and they're all over it at the moment plus a little cyno, I'm too ashamed to post a photo of it lol it's hard to get decent flow in a jar, i may transfer the shrimp over to a 3 gallon cube after i get one set up but they don't seem to mind the current setup at the moment. What are you going to do for flow in your jar?

Edit: i seen you're only going to have a air pump, that's what i did and it didn't work out so great, maybe you have a better way to do it but i couldn't come up with anything and it backfired slightly.

The plan now is to just keep it with a pump only. If it goes south, then maybe move to a slightly larger jar and use a small wavemaker for flow. Only if needed though, the small size it currently is just really attracts me and adds to my interest in it.

 

[pietervanwhelp]

Cookie jar update — I missed the nitrite spike entirely and somehow the cycle looks done. 8 days. Doubling checking tomorrow before I do anything.
I think the cookie jar is cycled. Either that or I've made a terrible mistake and everything is about to go wrong. Here's what happened.

I blinked and missed the nitrite peak​

Last update I had ammonia at 0.50 ppm and was watching for the nitrite spike. Skipped one day of testing as I was away from home — just one — and came back to nitrite still at 0. I never actually saw it peak. Ammonia has also dropped from 0.50 to nearly undetectable. And nitrate is now sitting at 10 ppm.

From what I understand, that's the cycle completing: ammonia up, nitrite up and back down (apparently while I was busy), nitrate accumulating as the end product. Eight days total from first fill to cycled, which from everything I've read is pretty fast — I'm crediting the live rock from my LFS.

Final parameters​

Parameter	Reading	Notes
pH	8.2–8.4	Stable throughout week 1
Ammonia	~0 ppm	Was 0.50 ppm, now fallen
Nitrite	0 ppm	Peaked and fell while I wasn't looking (maybe didn't?)
Nitrate	10 ppm	Cycle end product — expected
Salinity	1.024–1.025	Stable, only 1 RO/DI top-off during cycle
KH	10 dKH	Holding steady

I missed the Nitrite test on day 5 which is when I think the spike could have been.

Not moving yet — double testing tomorrow​

Even though everything looks fine, I'm giving it one more day before I do anything. Going to run a full test again tomorrow morning and make sure the numbers hold — ammonia stays at 0, nitrite stays at 0, nitrate stays stable. 8 days feels fast and I'd rather be certain than rush it. If tomorrow's results confirm it, then I'll consider the tank ready.

What's next — and I need your help​

Assuming tomorrow checks out, the plan is a small water change to knock the nitrate down, then start with a clean-up crew before even thinking about corals. This is where I'm genuinely unsure and would love specific recommendations.

I know the basics — nassarius snails for the sand bed, ceriths for the glass and rock — but I'm working with a 7.5L jar and I don't want to overdo it. What would you actually put in here? Numbers, species, order of addition — anything specific is hugely appreciated. What's your must-have CUC for a pico reef this size?

Perspective is strange here but water level looks low, its not, as you can see in pic 1. But there is a buildup of surface bubbles. Protein? Normal or something I should skim off?

Thanks for following along

 

[pietervanwhelp]

Cookie jar update — The cycle confirmed, the water change done, and the first livestock is now in the jar. A lot happened in the last few days.

The water change​

Did a 50% water change before adding anything — first one I've ever done on a saltwater tank. Had to recruit my wife to help, who was not thrilled that this was happening in the living room. I also started the siphon with my mouth, which I'm told is a rite of passage and also something I should never do again. Lesson learned.

The CUC​

Based on some advice and what my LFS had available, I went with:

• 2× Nassarius snails — sand bed crew
• 1× Cerith snail — algae on glass and rock
• 1× Peppermint shrimp

Acclimated them slowly with the floating bag method, adding small amounts of tank water over about 30 minutes before releasing them.

First impressions​

The moment they hit the water, chaos. The shrimp immediately started doing laps around the entire jar like it was auditioning for something. The nassarius snails shot across the sand bed in every direction and scaled the glass before burying themselves. The cerith played dead for a few hours and then climbed to the top of the highest mountain where he remains.

From what I've read, all three species are behaving as expected. The jar feels alive for the first time. Keeping an eye on the nassarius to make sure they re-emerge over the next day or two, and watching the shrimp to see if it comes out of its cave once it gets more comfortable. Parameters still looking good — testing daily still and will wait a couple of days before thinking about the first coral addition.

Still open to CUC suggestions if anyone thinks I'm missing something important for a 7.5L. Shrimp now, corals next.

 

[StartingATank]

The only thing for me that I think for CUC is that in my experience, my ceriths have really only stayed in the sand as sand sifters, so I would say get maybe a stomatella snail if you can or an astraea or trochus, all very good rock and glass cleaners. I have had my ceriths go on the glass before but never the rocks

 

[E. Hale]

Nice build! I'm in the process of setting up a 2 gallon pico as well :)

As someone who works at a LFS, something we always tell people is not to overstock the CUC, especially in the beginning, otherwise they can run out of food, so just something to be aware of. It looks like you have a solid base CUC, it might be a good idea to wait and see how they do once your tank ecosystem is more established with corals etc, up to you!

As for hardy coral recommendations, xenia is always a nice option especially with flowing movement, just in a smaller tank do ypur research and look at slower growing varieties (often other closely related corals that are very similar but not technically xenia because they are a different species in the same genus).

Another great option are some hardier Zoanthid and mushroom corals - slow growing, colorful, many different kinds available.

A final thought: adding some display macroalgae will further your biological filtration whilst adding an interesting variation, as long as you choose slower growing kinds not to overtake :)

 

[pietervanwhelp]

Nice build! I'm in the process of setting up a 2 gallon pico as well :)

As someone who works at a LFS, something we always tell people is not to overstock the CUC, especially in the beginning, otherwise they can run out of food, so just something to be aware of. It looks like you have a solid base CUC, it might be a good idea to wait and see how they do once your tank ecosystem is more established with corals etc, up to you!

As for hardy coral recommendations, xenia is always a nice option especially with flowing movement, just in a smaller tank do ypur research and look at slower growing varieties (often other closely related corals that are very similar but not technically xenia because they are a different species in the same genus).

Another great option are some hardier Zoanthid and mushroom corals - slow growing, colorful, many different kinds available.

A final thought: adding some display macroalgae will further your biological filtration whilst adding an interesting variation, as long as you choose slower growing kinds not to overtake :)

Great advice! I think I will definitely stick with the CUC I have now and not add more until I have some flourishing coral and see the need.

Ill definitely add some macroalgae as its been recommended by a few now and I like the way it looks. I've heard the xenia reputation quite some times so will be careful on those.

Are you sticking with only soft corals in yours?

 

[E. Hale]

I'm doing soft corals, red bush (gracilaria hayi) macro algae, and carpeting halimedia macro algae. Red bush is slightly calcified with slower growth, whereas carpeting halimedia is a fully calcified macroalgae with slow growth. Glad my advice is helpful :)