First, I would get more live rock, at least another 30 - 40 lbs... it will give you more surface area for denitrification and provide more area for coral to grow as well. Next, a really good skimmer is key. Several of us can recommend skimmers for your size tank that would do the job nicely. I've got a Aqua C Urchin Pro in my sump... oh yeah, I'd recommend a sump with a section to grow some marine macroalgae (like Chaetomorpha)... nitrates and phosphates are taken up by the macroalgae as it grows, continually reducing nitrates and phosphates in your tank. until you get a sump, you can take all of the bio balls, charcoal, and sponge out of your hang on filter and put in some chaetomorpha (i have some i can give you) as a temporary "poor man's refugium". The bioballs provide lots of surface area for bacteria to turn ammonia into nitrites, and nitrites into nitrates... but then you have a high build up of nitrates. Bioballs don't provide living area for the anaerobic bacteria that turn nitrates into nitrogen gas (that's what the sand bed and live rock are for).
If you go to the main Reef Central forum and look until the Reef Chemistry forum, and look at the chemistry articles, you can learn more about getting your nitrates down to zero and keeping them there.
