If your PO4 is high than along with water changes you could get a phosphate reactor. Do a search for them on the board there are a lot of proponents to these and there is one out now that is a real good deal.
Havent heard of any negatives from these reactors, just bear in mind the flow must be enough to keep media fluidized and sublte enough to keep the fine particulate matter from flowing into the tank. Ball or gate valves are perfect for this.
