Tank slowly dying


New member
Will try to give as much information as possible. I'm a freshwater person so am mightly flumoxed by this formerly thriving tank.

The tank:
100 gallons with 20ish gallon sump. Set up and happy for 8+ years. 2 clowns hosted in a large (18") toadstool mushroom, kole tang, yellow tang, 2 green chromis, 2 yellow belly damsels, 1 large and hundreds of small brittle stars, fire shrimp, banded coral shrimp and a very active collector urchin. Corals - large toadstool, large tree coral, zoanthid polyps, button polyps, torch coral, several leather corals, several chalice corals, many hammer corals and many many many red and blue mushrooms.

Water changes once per week 10-15 gallons using RO water from Ontario municipal water source. Premixed in large plastic garbage can (lid on) with circ pump and let set to mix until next water change. Matched to tank water for salinity - always 1.020 to 1.024. Seachem boxed salt. Moved cat litter box out of laundry room recently in case it was the issue. Adding bags of carbon to this reservoir too in a fit of trying ideas.

Nitrate and phosphate checked semi-regularly and always been negligible. <5 according to API test kits used. Other tests recorded below. These 2 never waiver. Test kits all API tests. Salinity measured with refractometre.

Tank thriving. Moved locations within same city March 2020. Everything transitioned well and no livestock or corals were lost - in fact we found a few we didn't know were still alive because they were hidden from view before. All practices stayed the same and all looked the same.

24 July - fire shrimp died. No other cause for concern
26 Aug - concern hammers closing and not opening up, some polyps being taken over by mushrooms
28 Aug - added banana wrasse, spotted goby and Xenia coral from quarantine tank (2 months in there)
5 Sept - red chalice dying, can't find goby. wrasse happy and xenia thriving
11 sept - torch dying, started upping water changes. Changed phos beads in reactor and added carbon, used Aiptasia-X to kill a few mushrooms near polyps to give them a chance
18 sept - calcium 280 ppm - start dosing calcium a few times a week
24 sept - torch gone, calcium 320ppm, 2nd dose of Aiptasia-x to get some aiptasia. Only used 2-3 syringes worth
1 oct - leathers weak, KH 8-9, calcium 360
2 oct - damsel died (at least 6 years old), snail died (at least 4 years old)
6 oct - new RO unit and new inserts, most button polyps gone.
10 oct - calcium 400, KH 9. mushrooms not opening fully
8 Nov - calcium 440, KH 9. Wrasse missing, no body. Toadstool staying closed. Most corals closed most of time or fully gone. Other fish acting normally and feeding fine.

Since tank move: no change in food or feeding regime. No change in algae. No direct sunlight on tank. No change in cleaning chemicals used in tank room.
Sump - refugium - thriving caulerpa - no change. Protein skimmer actively working - no change. No change in circ or return pumps. Lights replaced November 2018 and regularly wiped to keep clean. No electricity felt when servicing tank. Temp of tank 25*C with no change.

I'm at my wits end. What else do I test? What else do I think about? What am I missing?


New member
You have not mentioned your substrate, is it a deep sand bed by any chance? Only thing that comes to mind if swapping tanks or moving. They don't take a disturbance well if in an old mature tank.


Premium Member
Regarding the sand if it's an 8 year old system I would replace a lot of it with new sand. I wouldn't disturbe any of the aquascaping to get it all out but just would replace what's easily removed with siphoning.

It sounds like the GAC is just set in bags in the water flow which doesn't work too well, I would use it in a reactor so there's a good consistant water flow forced through it.

I would stop carbon dosing if your doing it as well as stop dosing any amino acids.

One thing I've found useful if fixing these problems in systems that have been running for years like your is running a diatom filter. Corals are constantly shedding their mucus coatings as the bacteria in it ages and it seems to gradually alter the microbial composition over time. Water changes help keep slow this shift but may not prevent a slow huiild up (I've heard 2nd hand Jason Fox does frequent large water changes). Diatom filters are a hassle to set up and run and they clog very quickly but as an alternative you might try a series of very large and frequent water changes.

Michael Hoaster

Registered Seaweedist
Premium Member
This sounds kinda like 'Old Tank Syndrome". This is a general term with many possible causes. I found an article that goes into several.


Think about all the stuff we add to our tanks over the years. Now think about all the ways we export stuff from our tanks. It would make sense that some of it gradually builds up. And then there's Allelopathy - chemical warfare, between corals, algae and even bacteria.

Moving the tank probably upset the delicate balance, and now things are out of whack. I agree with Timfish that efficient charcoal usage is called for. Oldreeferman also brings up a good point about your sand bed. Things tend to accumulate in them. Moving the substrate would release a lot of unknowns into the water.


New member
Thanks for the replies. The carbon is in a carbon reactor in the sump. I do know there's a bit of a delicate balance of not enough carbon to make it worth while, and too much it gets clogged. I will recheck and see how it works.
The bed is crushed coral aragonite and isn't terribly deep. It was regularly siphoned before and after the move and didn't have any deep areas to have anaerobic pockets.
We are going to try several very large ie 50-60% water changes to clear things out. My experience with old tank syndrome in freshwater is that it reflects in the water chemistry we can see - ie usually it's the KH that drops fast, which hasn't happened here.
I've got a friend with a diatom filter I've borrow before. That' a great suggestion and will try that too.
Thanks everyone


Cloning Around
Staff member
RC Mod
Premium Member
You mention "changing the lights" in November, 2018. What do you mean - new bulbs, or new lighting system? What type of lights are you using?

I noticed a few other things - nitrate and phosphate NEED to be present for corals to grow (generally nitrate at 100x higher than phosphate - ie, 0.02 ppm phosphate with 2 ppm nitrate). Obviously, you caught your very, very low calcium, but that could be a contributor.

Finally, you mention your SG at 1.020-1.024. Depending upon timeframes, that a pretty large swing (20% change).

All of the above could have an effect on your tank's inhabitants, especially the corals. Lots of light but not enough calcium could cause problems for stoney corals (of all types). Adding back calcium while not having enough nutrients to support growth with the available lighting can also result in coral failure - especially when you have other competitors for the available nutrients (like your caulerpa).

It could also be the "old tank syndrome" catch-all mentioned above - it seems to happen, but without anyone being able to pin a real reason on it.

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Sinn Sage

Look at what has changed since you moved. Could it be your new water source? Maybe that stuff coming out of your faucet isn't really that healthy. Ask your local water supplier and see what are chems that put in there.


RC Staff
Staff member
Can't hurt to get an ICP test done. You may find something out of whack there.