heres a link for more info:
http://www.aloha.net/~hqf/indexdondenitrator.htm
basically all Im doing is using a 16" piece of 3" PVC, running a coil of 1/4"airline tubing from the bottom of the reactor till an inch from the top. In the cavity of the reactor will be bio-balls( or variation). As the water slowly runs through the coil it loses O2 and and thus only nitrate eating bacteria are left from that point all the way through the bio balls, thus allowing only nitrate free water to drip back into the tank. Dont know how well it will work, and doesnt mean Ill do less water changes, but even to work a little bit would be beneficial.