I have a 55 gal tank with 2 goldfish, 7 cherry Barbs and 1 bristlenose pleco. 2 Marineland 350's for filtration. Substrate is a mixture of Petco "sand" and gravel. I, for the life of me can't understand why I cannot lower the ammonia levels. Ph - 7.0, ammonia stays at 4 ppm or more, nitrite - 0...