I always test my water twice a week and do water changes twice a week. Usually 50% WC. I've noticed that my ammonia and nitrites stay at 0 but my nitrates are always over 80ppm. When doing a change I test the water and they are down to about 40ppm. In a couple of days they are back to where they started. I only feed what my fish can eat in about 1min and feed once a day. Any suggestions?