You must b feeding alot. Try stepping up your water changes to 15 gals per week and cut back on the amount of food you give at one time. Are you running a skimmer? Is it a reef tank or fish only?
I think API nitrate test is not good, I used to get a reading of 80-100 ppm with fresh API and only 20- 25 ppm with Salifert, still high but a big deference. Try another test kit.