Water Chemistry

Hi,

I just started with this fantastic Hobby, I got an 90 Gal Tank with a 30 Gal sump, i use dry/wet, I have 20 Lbs of live rock, and 6 fishes ... This tank is 5 month old, Ammonia, Nitrites, are 0 ppm, Ph 8.2,, but nitrates are some time 20 ppm or 40 ppm.... i do 15% water change every 2 o 3 week... I feed them 1 once a day, and i make sure the eat every thing and try not to over feed...

any hint on how to have better control of nitrates?

Saludos,