I bought a used tank 92 gallon, when I brought the tank home I took half the water. I filled the other half with rodi water. I tested the nitrates were close to 160, the next day I did another big water change and the nitrates didn't come down much. I replaced the pre filter pad and cleaned the sponge in the sump. Today I did another water change 35% and still didn't come down much at all. I tested with 2 different test kits and the same reading. Is there something I'm missing? Can there be something in the rock (i have about 100lb).