my friend has a 90 liter aquarium. It has been running for a while now maybe 5 months and all his fish keep dying. It is baffling because one day they look ok the next they are dead! He has 3 guppies, 3 black widows, 2 gouramis 1 clown loach remaining. they are all still young fish and are not really that big. the fish that have died include 2 juvie red rainbow fish, 2 black widows, 2 guppies, 1 clown. the ammonia was at .25 nitrate 20-40, ph 7 not sure about nitrite temp 23-24c and 20% water change every 1-2 weeks. Alot of these fish are meant to be hardy so we do not think it would be water. they die very suddenly no sign of illness. i thought maybe lack of oxygen? We have been changing the water and adding stability because we are not sure what else to do. also the tank has no light or lid. any help would be appreciated.
Any amount of ammonia is toxic. Nitrites are just as toxic, if not more so, than ammonia, so you should invest in a test for your nitrites.
Not allowing the tank to cycle before adding the fish is a major contributor to their loss. Also, I suspect stocking and incompatibility issues played a part as well. A 90L tank is about 23 US gallons, so there are some fish that you mentioned that will grow too large for that size tank. There are also fish that you mentioned that should be kept in larger groups, and you only have a few of those; the school isn't big enough. This can stress these fish out (which causes sickness and death), especially in tanks with other fish.
I would read up on the nitrogen cycle, and purchase a nitrite test. Ammonia and nitrites should test as 0, and nitrates as less than 40 parts-per-million; less than 20 ppm is even better. Definitely up your water change schedule to 20-30% every 1-2 days until your water tests consistently come to these levels. Make sure new water is conditioned and the same temperature as the water in the tank.
Check out this website for stocking and compatibility. It is a great resource.
As for oxygen, an airstone with an air filter can be hooked up, but the surface agitation from a filter should be sufficient. I am personally fond of bubbles, though.
I'm not sure what you mean by "adding stability?"
A light isn't too important unless you have live plants, but a lid is very important. Not only will it keep things out of your tank, it will help your fish stay in!
I Hope this was helpful!
Fishes dying at night is a mystry issue.
but you have to get into the problem.
You mentioned that the tank is without lid, it may be that someting from outside falls into the tank, say lizard pee etc, which contaminated the water, or may be that at night temperature in you area drops which leads to temperature below 20deg centrigrade, which can be fatal for trophical fishes.
are the fishes eating ?
you can put an thermostat heater say 200W, to your friends tank. Temperature around 26 deg centrigrade will be fine for the fishes.
keep us informed
thanks for replying. The tank is cycled is has been running for almost 6 months some of the original fish are still alive. Although your answer makes sense it does not really account for there red rainbows death so quickly. The red rainbows were in for a few days as they are hardy fish @ current water parameters i think they would survive.I might go buy a nitrite test kit see what that says.
That website is really helpful thanks.
*Stability is a seachem product for good bactiria.
it could be not having a top on. Should he cover the tank at night with something maybe? they were eating but now not really. I have a feeling that the tank will be empty soon. The weirdest thing is it was 2-3months between the old fish and new fish he bought. Most of the old fish died and some are still alive. Just talked to friend many fish are nipped fins? tank down to 2 gourami, 3 zebra danio. all doing fine!?! 5 days and 11 fish have died!
which fishes are not eating ?
which fishes have a nipped fin ?
I think an aquarium hood is necessary for a tank, its not only restricts outside things from getting in, but also keeps fish from jumping
all the fish that were not eating/nipped fins are dead now. at night gonna put a towel over until he can get a new tank also turned up the temp to make sure there are no deadly drops overnight.
try to get a hood for the tank, put a net instead of towel.
be a member to get more info, keep us informed.
I agree with Suvro that a lid is very important.
Most problems with fish stem from water quality, and your friend does have an ammonia spike. What is causing the spike? Hardy or not, ammonia and nitrites are still toxic. Death from ammonia poisoning can happen from almost instantly to a couple of months after exposure. Ammonia poisoning weakens the fish's immune system opening it up to other infections.
I think the tank may be overcrowded, and the biological filter can't keep up, allowing ammonia to buildup. I still suggest increasing the water change schedule to 20-30% every 2-3 days to reduce the ammonia.