Yes, you said nitrates are 0, but nothing about nitrItes. Fish waste goes through three stages. It comes out as ammonia. Then, it turns from ammonia in to nitrItes, and finally from nitrItes into nitrAtes. Ammonia and nitrItes are both very bad for the fish. NitrAtes not so much, and are kept in check by regular water changes. if your nitrAtes really are 0, then I would suspect your tank has not cycled yet, and perhaps your nitrItes are very high. A tank with 0 nitrAtes is very suspect unless it is heavily planted.Nitrates are 0