dying tetras

Question
QUESTION: Hi, I have a 300litre tank in which I had 20 tetras, 3 kuhli loaches, a couple of catfish, 2 indian glass fish some guppies and their fry and a few small angels.
Over the last 2 days I have been loosing a lot om my tetras and have also lost a couple of angels, I have never had a problem like this before.
The tank has a ugf and a 500 litre/h canister filter.
Water is 0 amonia and 0 nitrate.

ANSWER: Hi Dave,
Massive fish losses like these almost always point to water quality problems. Are you certain you tested the water correctly and the test kits are truly accurate? Also, how is the nitrite level? Nitrite is almost as toxic as ammonia.

How was the fishes behavior just before you lost them? Were they breathing heavily and otherwise acting stressed? Or any other abnormality?

Just before they started dying, did you do anything different to their aquarium?

Sometimes you just get 'bad stock' from your local fishstore and this -could- be part of it depending upon how long you've had them.

I'm definitely thinking a water quality problem here. Otherwise it could be an illness that doesn't display any obvious symptoms on the fish...but I do think that to be unlikely.

Let me know how things turn out.
Hope this helps!
Karen~

---------- FOLLOW-UP ----------

QUESTION: Hi Kareen,
I have been paying very close attention to the water quality and how the fish have been acting now and the fish dont seem to be as ravenous at feeding time now.
I have an angel that is about 2.5inches high and he seems to have lost his appetite.
Since I asked my last question I have also lost about 10 more tetras, an indian glass fish and 2 small angel fish.
My guppy fry seem ok, one of my kuhli loaches seems exhausted, I can't find my other kuhli loach and my biggest kuhli loach seems to be loosing his brilliant bright orange and blown stripes to a dull colour.
I have noticed what looks like an air bubble behind the eyes of my other glass fish, not to sure if this is just its internals or something worse.
My biggest angel has a red tinge around his gills and I have started to notice it's veins are becomming more obvious.
I couldn't notice any signs of stress in my small angels or indian glass fish prior to them dying.
I have crushed granite as a substrate and it hasn't bothered any of the fish before, I have a few rocks I found at the beach covered in java moss.
I have a few other plants but I can't remember their name.
The Ph was a bit over 8.4 tonight when I tested it, the amonia was zero and the nitrite was also zero.
Any help you could give me would be very much appreciated as it is starting to get stressing.
Cheers
Dave

Answer
Have you tried big (50-75%) water changes yet? The red veining in the fins/ red coloration around gills, loss of color, and listless behavior you mentioned are extremely obvious symptoms of some kind of water pollution trouble. You say you have tested the water...But its always possible for the tests to be false and even certain water conditioners can cause false readings. So that's why I think it would be best if you started doing daily 50% water changes ASAP. Always, of course, treating the new water with water conditioner and insuring the temp is close to or just a bit warmer than your aquarium.

How long has this tank been setup?

Water changes are the best I can recommend right now at the moment. We can't properly diagnose any kind of illness yet and recommend a medication until we are for certain this is indeed an illness and not symptoms of bad polluted water conditions.

Best of luck!!! I do hope all goes well!
Karen~