Nitrate levels

[jordsy]

I bought a 90 gal tank already set up. We used the water he had and 90lbs of live rock. Although we used brand new sand.
Its been up and running for a month and a half and ive been testing quite often. I just found out my test kit for nitrates was way off. the new test kit says my nitrates are about 25ppm. Ive heard this is high so i did a 25% water change and a week later did a 50% water change....
My nitrates still have not come down much and i tested the source water and there is no nitrate level expressed.
How can I get these nitrates down???? I dont over feed but I am also gettin some algae growth on the rocks now...