Not sure of high nitrate levels, but I just did 50% changes because everywhere I went they suggested that as a standard procedure. Most LFS told me never to do more than 50%, but that they should be done weekly because it was fest for the fish.
I've honestly never tested my water for nitrates in the 6 1/2 months it's been setup. As it is now I feel I'm underfeeding. Everything is gone within 20 seconds, whether it be pellets or frozen cubes; nothing touches the sandy floor. I do feel it is a bit over-stocked because of the festivum, but I'm planning on removing him soon anyways because he is destroying my plants.