Troubles with my fish.

Question
I had three fish in a fish tank that could only hold about three, they are fairly small fish. Two of them died, and one is left. I feel as if he is stressed, he isn't acting the same, he isn't swimming, he sits still, I don't see him eating all that much, and I don't know what to do, I looked up on google about stressed fish and ill fish, and possibly could he be stressed or ill? Also, since he is the one left, I have bought a larger tank with a heater and filter, and I am planning on adding more fish of his size into the tank, I don't know if he will get more stressed, or if it's good to put him in with other fish, or just because its a whole new tank! Please help me with this problem, thank you!

Answer
Hi Kate:  Lets start with your old tank... try adding some oxygen to the water... you have described to me either a chemical poisoning or anoxia... lack of oxygen.  Have you had the water tested for ammonia or nitrate?  Also when and how do you clean your tank.... Tanks with small volumes of water go bad quickly... Make sure you are cleaning the gravel when you change the water... in fact I would do that immediately... 30% water change and clean the gravel.

NEW TANK... set it up and let it run for a week... take the water in to get tested when you go to buy your fish... if the water levels check out ok then buy your fish... if your little fish is ok... add him to the tank when you buy the others... or even before so he can check the place out.  They will all be stressed but that should pass quickly... feed them well but do not over feed them... no more then they will eat in three minutes of time.... I hope this helps... but feel free to ask me more questions if I have not covered everything.... dave