I can not get my nitrate levels below 10ppm. All the other tests are 0 ammonia, 0 nitrates, 10nitrates, 0 phosphate. The tank is a 75 gallon, with 80 lbs of live rock, crushed coral/sand, I do at least a 15% water change every 7-10 days. There are 2 fluval 305's without the filter pads those...