Sorry to hear about the nitrate problems. I just went through a battle of my own. It may be wise to do a 25 to 30 percent change now and another next week to really put a dent in the nitrate levels. I am fairly new at this, but am just passing on some advise from my own thread regarding nitrates. I was also getting skewed nitrate results because of a poor quality test kit. After a couple of big water changes and a few smaller weekly once, (about ten percent) the nitrate levels will hopefully calm down. Also be very careful to not overfeed your fish, maybe drop down to every other day with smaller portions for a while.
Good Luck,
JD