so i have a 5'x2' tank with 2 juvenile stingrays in the tank. the tank has been setup for while and is established. the rays have a 5-6" disc. they are young. currently only fish in the tank. I currently have a fully loaded fx6 on the tank. I am having a battle with ammonia right now. It is currently between .25 -.50 ppm. i have been doing twice daily feedings and a 20% daily water change at night. Any ideas why i still have ammonia?