Oncology Lab Zebrafish Tank Problems

Question
Hi Chris-

You've proved so helpful to so many on here, i thought i'd ask a
question. I run a small oncology laboratory zebrafish facility that
houses several tanks segregated by sex. There is some turnover
from the fish, but the population i have right now is stable
without any death in several weeks. One of my tanks that houses
male fish has been having problems with chemistry levels. I've
noticed that there seems to be a film on the surface of the tank
and the tank water is slighly cloudier than it's counterparts.
Furthemore, levels of Ammonia, Nitrates and Nitrites have been
elevated. At first, there was a large spike and i ended up
relocating half the fish to other tanks to reduce the burden.

They were not housed in a new tank, no new filter or biofilter
(same biofliter for nearly 5+ months) but now, despite small
daily water changes (15-25%), daily Amquel+ and Ammolock, i
am still having problems with high ammonia levels, although
Nitrates and Nitrates are within the tolerable range.

What could be happening? The fish don't look sick, none have
died and i have confirmed the readings with dipstick and with
kits using drops and they're in agreement. No death has
happened and they are still breeding with the females when
needed. I am thinking that the biofilter isn't working well, so i
am considering starting a new tank and trying to get a biofilter
seeded so i can perform a 'transplant' when needed.
Furthermore, i've changed the filtration apparatus (but kept the
biofilter, of course) to rule out mechanical problems. And i've
taken to adding weekly stress-zyme to attempt to add beneficial
bacteria to the tank, but this hasn't seemed to help yet.

FYI: the water changes are done using special reverse osmosis
pure laboratory water and so there's no way chlorine could be
affecting the biofiltration. Chlorine and chloramine levels are
zero. Tanks are all eclipse system 3 tanks (3 Gal).

Ammonia levels: 2-5+ by two methods
Nitrates: <40
Nitrites: <1.0
# fish: <10 in the tank

** All other tanks are: 0,<20,0 for the above

Thanks so very much. I apologize for this lengthy question. Take
care!

-Geoffrey

Answer
Hi Geoffrey;

There are just too many fish for the little tank to handle. The recommended population for any tank with small fish like those guys is one inch per gallon. However, it is possible to have as heavy a load as 2 inches per gallon, especially with the water changes you are doing. (I wish I could get every hobbyist to be that diligent!) Even if those little guys are small, it puts their population over that.

I am puzzled at why the other tanks are doing okay. If they have the same fish load and the same filtration, etc., you would expect the same result. But, every tank is different and you just never know for sure.

Keep making those water changes and move some of the fish to another tank. Hopefully it will balance out once it is more appropriately stocked.

Followups welcome!

At Your Service;
Chris Robbins