i have a 36Lx12Wx14H tank. It currently has one 3" gold piranha and a 5" plec. I have a marineland penguin 150 power filter and an undergravel filter. Water is clear and the fish seem to be happy. I just recently tested my water and everything is fine nitrite 0, ph and everything is ok, but my nitrates were really high like 100ppm. so i did a 75% WC and about 15% WC everyday now for a week. They have gone down a little but not much still right about 80ppm. What do you think the problem could be? Have i just not given it enough time to go down? Any help would be awsome. oh ya tank has been set up for about 5 months