Take the concrete brick out of the sump...water changes never will help if sand bed is nitrate trap...I had a denitrator...will work...however, using vodka/sugar/vinegar and MB7 will allow you to use your whole tank as a denitrator (a good thing)...In creasing flow, reducing fish load, reducing feeding, increasing clean up crew, biopellets, all good ideas....but again, if tank is being "abused" by overfeeding and over"fishing", nothing will help long term...I was an abuser...had to finally resort to tear down and get rid of rock and sand...
DrMerle