I have a 90 gallon aquarium that is having a problem with nitrates. The other chemistry, ammonia, nitrites, etc are very good. But my nitrates are sometime higher than I want. Anyone know what to buy and where to buy it to help me keep those nitrates lower? I am using a marineland c530 along with a HOB Tetra filter, a UV and a home made bio filter. Some suggestions would be greatly appreciated. Thanks.