I have a 29 gallon freshwater tank that has been up and running for more then 2 months. It is currently stocked with 2 small Angels, 2 Pictus cats, 3 Bloodfin Tetras and 3 Cory Cats. In the last month I have lost 6 Swordtails. My filtration system consists of an undergravel filter powered by a powerhead and a Fluval 203 canister filter. I have tested my water daily and have 0 ammonia, nitrate has been reading around 30 to 40ppm (safe) and my nitrite is off the chart. I have been doing 25% water changes every other day for the past 2 weeks using ro/di water and it will not come down. What else can be done to bring this level down? I thought it could have had something to do with my former Penguin Bio Wheel filter not being sufficient so I changed it back to the Fluval Canister Filter. Please help, this is driving me crazy......And to think I want to start a Marine Aquarium!!!