Water changes. I would start right away with the water changes. THEN, find out what is causing your high nitrate problem. Otherwise you will be doing forever. Nitrates come from ammonia and nitrite reactions. Do you have any uneaten food laying around in the tank? Could be a few possiblities as to why they are so high.
But, start with the water changes first and get them down as low as you can.
 Malkore, you beat me to it...LOL
. I was typing this while you were posting.
125 Gal-35 Gal Wet/Dry-Tetra-Tec PF500 HOB
(3) AuquaClear 301 Powerheads-600 GPH
Automatic Tempature Controller
5-6 Inch Sand Bed-200+ lbs of LR