Help Asap!!!!

[pre-2k]

On Mon. Feb. 12/06 I did a 20% water change on my 120gal. I checked my ammonia, nitrate, nitrite and ph levels. My ph was at 8.2; nitrite is at zero; nitrate was at 20ppm; and ammonia was at 0.5ppm. A couple of days later I did another water change of 10%. My levels all remained the same. And today i did a 50% water change and checked the levels and they all are still the same. I did not add anything new to the tank in the last 2 weeks and feeding is the same (once a day), the only thing I added new was reef roids.

Tank Info:
-120gal
-170 + lbs of LR
-3-4 inches of sandbed
-1 tang, 2 yellow wrasse, 1 dragon goby, 1 zebra goby, 1 lawnmower blenny, 1 per. clown, 1 blue damsel, 1 ladder goby, 1 horseshoe crab, 1 molly, 1 leopard wrasse
-4 assorted anemones, star polyps, fox coral, pulsing xenia, button polyps, toadstools, assorted mushrooms, flower pot
-inverts
- tank has been established for 5 months
-also have a 30gal sump, protein skimmer, cannister filter, overflow box
-lighting-2-175w MH, 2-VHO actinics, 4-40w Pcs

Can anybody help me solve the levels of nitrate and ammonia in my tank???