I can't seem to get my nitrates to go down. I started with higher than normal nitrates in my fish only tank. I removed the fish, all the gravel and crud in it, cleaned the tank walls filters, and did a 25 gallon water change (75 gallon tank). I added new live sand (40lbs) and 80-90lbs of cured live rock. I also added a new sump. I keep doing water changes and the levels go down slightly then rise back up. What am I doing wrong?

Wow, it sounds like you basically started over. If that is the case you might be going through the standard cycle when a tank is first set-up. Do you have any fish in tank right now? Also you mentioned a filter, what kind? Sponges filters and bioballs trap organics and typcially cause all sorts of nitrate problems.