When did you set up this tank? Do you know about he nitrogen cycle? Please read this:
http://www.fishlore.com/NitrogenCycle.htm it's very helpful and will help you keep happy, healthy fish.
You should invest in a test kit. The API master kit is what most people on this forum use. You need to check ammonia, nitrite, and nitrate readings regularly. Fish don't tolerate any ammonia or nitrite in their water very well and will die from prolonged exposure to them. I don't really know what's wrong and somebody else can help you out more but I'm thinking that the fish are being affected by the ammonia and nitrite in your water and maybe even lack of oxygen. You need to do 50% daily water changes until your tank is cycled and you might want to put in an air stone to increase the oxygen in the water. Make sure you use a water conditioner to remove chlorine/chloramine from the water when doing changes.
EDIT: looks like I've been beaten to the punch again...
