fish suddenly dying and no idea why..help?

Question
Hi Karen,

My fish are suddenly dropping dead and I have no idea why! Could you help me?

I have a 20 gallon tank; as of yesterday there were 3 dwarf gouramis (2 flame, 1 blue), 3 glassfish, 6 cardinal tetras and 1 clown pleco in it.

The cardinals and the two flame gouramis I just got on Sunday, the other fish I've had for a few months.

Last night everyone was fine except for my blue gourami, who started sitting at the bottom of the tank and being listless after night-time feeding (but before and during he had been perfectly normal, eating and swimming about).

This morning I woke up to discover 3 of the cardinals belly-up and my clown pleco was dead too, and the blue gourami was the same as last night. I came home from work to find him dead as well. I tested my water and everything came out fine, 0-0-40, pH at 7.8, but I did a 25% water change anyway.

I did a lot of research and read that the signs my blue gourami was showing might have been internal parasites? But the odd thing was that he only looked sick for half a day..not even enough time to treat him.

The dead fish have no signs of disease - no missing fins, no patches, bumps or sores, no nothing.

The fish that worries me at the moment is one of my new flame gouramis - he spends a lot of time swiming up and down the glass and trying to swim out of the tank, generally, and doesn't seem too attracted to food.
Could this be a bad sign?

The other fish all seem fine and unfazed, but I'm really freaking out because all my fish simply drop dead without any warning. They seem happy, I go to bed. I wake up, hello dead fish.
(This happened to me once last month also, with another flame gourami, that I got at the same time as the blue - about a week after coming home with me, he was suddenly dead. Fine one day, dead the next.)

Do you have any idea what could be wrong?

Thank you so much!

Answer
Dear Kim,
This is a tricky one. Fish suddenly dying for no apparent reason even after poor water quality has been ruled out needs detective work. It is well known that fish that die after only a few days of owning could be a result from previous petstore trauma or even an unknown illness.
You can run into bad batches of fish at fishshops many times. With all the stress of being shipped and unknown possibly bad treatment at the store can be too hard on their systems to where they eventually succumb. They could have had some internal problems, but it's so hard to diagnose
You could be getting fish from a bad source--but I'm only guessing. What I would try is switch petstores if you can and see how a different petstore's fish turn out.

This is only a guess as well, but the Flame gouramis and the Blue Gouramis could be sensitive to your somewhat high nitrate. Ideally you should be below 40ppm. But many fish can stand it higher--but really shouldn't be subject to it. This is just a thought.

The new Flame gourami could simply be seeing his reflection in the glass. It might take him a while to take food at first. But try to make sure any poor water quality isn't driving him to try to get away. ;-)

But I don't believe you are doing anything wrong. Just be careful and try to switch petstores.

I'm very sorry to hear you losing all those would-be wonderful fish. Let me know if you are still having trouble even after switching petstores whom are retailing fish from a totally different hatchery than what your usual petstore does.

I really hope this helps! Feel free to write with anymore concerns you may have...

Best wishes and Happy fishkeeping!
Karen~