What causes my nitrates to keep being high after water changes? The tank is well established since Oct. of last year, param. 0,0,40-80, tap n03 5.0 . Did a 50% W/C yesterday, today nitrate back up to 40-80. 40 long fish: 6 platys, male/female DG, 6 rummy nose, 6 peppered corys and 2 BN, lots of plants, driftwood and slate. Any ideas? on a side note why does it keep saying that my title must contain 10 or more characters, I had to add two question marks to the title just so it would take.