Have you tested the water? Did you check pH, ammonia, nitrite and nitrate? That's usually where the problem lies, and especially in a newly established system, you need to test it DAILY. Change about 30-50% of the water whenever ammonia shows up and add beneficial bacteria to the tank after every water change!
Of course, you need to be sure that the tank is big enough for the amount of fish you have, so... how big is the tank and how many fish do you have?