I have one on my tank. I didn't "feed" it to get it started. Mine works purely on a gravity feed, and then dumps into the top of my sump. Only time I gravel vac is to clear up debris. Nitrates never get higher than 10 ppm MAX.
The hardest part of maintaining a Denitrator is flow adjustment. If your flow is too fast, Oxygen isn't propperly depleted, and the filter won't work. If you go too slow, the water in the filter will stagnate, and the possibility that hydrogen sulfides might be produced. Youll know when this happens as the odor of the water leaving the reactor will smell like rotten eggs.