I take care of my father in laws jbj 45gal. For whatever reason the past few months he has had high nitrates and no matter what I do it won’t go down. I finally got it down from 160ppm to 40ppm but it won’t get any lower. What do y’all recommend? He has a protein skimmer and it’s doing a great job. It’s a mixed reef with softies, lps and sps. Has 6 fish; 2 damsels, 2 clowns, 1 coral beauty and 1 yellow watchman. Only corals he has lost is frogspawns and hammers nothing else and hasn’t lost any fish. I do water changes every two weeks of 25%. Also only feeds tank once every two days. Any ideas? Thanks!