Advice on Nitrate/Phosphate problems

[molybdenumman]

In the past two months (maybe more), i have problems with nitrate and phosphate levels in my 90g reef tank. Phosphate levels have ranged from 0.5 - 2.5 ppm and nitrates have ranged from 10 - 25 ppm. I am at my wits end trying to bring the levels down. I have tried nitrate and phosphate sponges with limited success (phosphates dropped for a little while, but returned). I have done two large scale (>20g) water changes recently with no success. I have also cleaned out my sump and i clean out the foam filters bi-weekly.

Deionized water and salt mix have shown negligible levels of both anions. I feed the tank freezedried bloodworm and mysis shrimp (washed before addition to tank). I will also feed flake food (prime reef, cyclopeeze flakes). Over feeding shouldn't be a problem as i only feed every second day if i remember. I give phytoplex (2 capfulls) 1-2 week.

Can anyone suggest any alternatives to getting those levels back to a healthy range? What other sources of phosphate and nitrate could be introduced accidently?

Thanks