I have a 28 gallon Tetra curve freshwater that consistently keeps the ammonia level between 5 and 1 no matter what i do or use. I've used Tetra Safe Plus; Topfin Ammonia Remover and Seacham Safe. I've changed 25% water. The tank is only about a month and a half old. It sat for 2 weeks after the initial filling before putting fish in. I started with 6 fish and it's down to 1. What else is there? TIA