I have a 125 gallon tank with a 55 gallon sump/refugium. For a skimmer i have an asm g-3. My fish are: 2 chromis, yellow tang, blue hippo, powder blue, 1 lyretail anthia, angelfish, damsel, scissortail goby, and three clown fish. My nitrates are at 20 but every time i do a 30% water change, nitrates still stay at 20. I have a r/o unit and it reads 004 on the T.D.S. What could be the problem be since the nitrates dont go down. All the corals and fish are doing fine. Any suggestions?