Well, e-cat seems to think you were recently asking about cycling, so my money's on ammonia poisoning. No accusations here, though. A lot of people are misled by
LFS employees into buying a tank and fish on the same day!
To tell if it is an ammonia problem, buy (or use, if you have one) a test kit that tests for ammonia, nitrite and nitrate. If the ammonia is above 0, or .25 parts per million (
ppm), it is almost certainly the source of your problem.
If this is the case, do a water change fast, or add bio-spira (highly recommended by all ppl on this site!). It will cycle your tank for you quickly. It's not easy to get, though. So look into it on the web, or post a topic asking for Bio-Spira. Allivymar says it's tough to get Bio-Spira in Canada, so you might want to start asking now.
Also good for neutralizing ammonia are the packs that Aquaclear (and other companies) make to add to your filter. This should help.
(IF ammonia is your problem!!!)