If youre looking for a fast plant to suck up those nitrates try "asian ambulia"-Limnophila sessiliflora. I started off with 6 stems in a high tech tank and BAM, 20-30 24+inch stems. It's considered a weed!
Another thing to think about is you may be over feeding them. When i just started my fish tanks i would test my water and the nitrates would be ridiculously high. But i realized after a while that i over fed the fish. You could also try using a feeding dish.