I have about 30 Chichlid fry's. Everything went well for a month. They are about an inch now. And they have started to drop dead. One dies every other day with same behavior. They swim erratically and breath heavily for half a day before they die. Amonia 0, nitrite 0 and nitrate 20-40 ppm. Change 30% water almost every day. Feed twice a day. Can some one share any wisdom on this situation?