I have an established 90 gal.tank that has been running for a few years. My problem is I can't seem to ever get the nitrates to drop they are always over a hundred.I have about 125 lbs of live rock (figi) and 40 lbs of live sand 20 of which I added a week ago. I have a skimmer, two powerheads and the main pump circulating the water. I put a nitrate remover (purigen) in the sump did a 20% water change two weeks ago and a 15% this week and they haven"t dropped. There are only 4 fish in the tank a power blue tang a yellow tank a domino and a clown. Took out all the bio balls a month or so ago. Does anyone have any ideas how to solve this problem. Thanks in advance for any help