Well, you need to get yourself a simple API master test kit. You need to know where your ammonia, nitrite, and nitrate levels are. Your fish are probably stressed from elevated ammonia and nitrite levels, which is toxic. Normally, we cycle our tanks for about a month with an ammonia source to build up the bacteria to ensure this doesn't happen prior to adding fish.
Since you already have fish, there isn't a need to panic. Pick up an API master test kit so you can test those 3 levels. You want to keep ammonia and nitrite at 0 and nitrate under 40. Those are safe levels for your fish. To keep those in check, when you see them elevated do a water change.