High nitrate question....help!

[bavass]

I have a 55g tank that has been up and running for awhile a few years. I have 55lbs of live rock. 2 powerheads in tank, turbofloater 1000 skimmer, mag drive 7.5 in sump for return, Rena Xp2 canister filter with activated carbon, phosphazorb and the standard filters all is replaced every 30 days. 2 small pumps in sump for circulation. The only thing I still have living in the tank is a star fish. Was trying to get nitrates under control before adding anything. My nitrates are off the charts. I was doing some reading and it might be beacuse I have about 4 inches of sand. Was thinking of taking out about 3 inches of sand and leaving just about 1 inch of sand. Then there is less sand for oragnics to be trapped in and be easier to clean. Should I do that or should I remove all the sand and put new in? Water changes are not helping. I have been doing about 20g a week for months almost a year now and still no drop in nitrates. Does anybody know if this will help? Was also thinking of buying some Reef Rock and adding that and make some more live rock about 50lbs is what i was thinking of adding rather than adding live rock for 3 times the price. I also use a tap water purifier when doing water changes.