Also, to protect your fish from nitrIte poisoning, add one teaspoon per gallon of plain salt (canning/pickling salt) even iodized salt will work in a pinch, as long as it doesn't have caking agents/yellow prussiate of soda. Aquarium salt is just a waste of money, and when you have ]V[onster tanks, just doesn't cut the mustard!
The salt will out-compete the nitrIte in the fish's gills (hm...fishs' just doesn't look right...need to ask my english teacher about that on...) anyway, you don't want any nitrItes, but they are necessary in a tank that needs to cycle, so the best thing to do is to protect them from nitrItes, until they begin to read 0.
NitrAtes can climb and stay high even after frequent water changes, be sure you are BANGING out your filter sponges/pads, because built up gunk in there can lead to high nitrate levels that can't be contained with water changes alone. Also, vacuum your gravel, this will make a huge difference. Do one half of the tank one week, then another half the next week, and rotate that way.
Good luck, and remember
If your nitrAtes are 180, and you only change 50% of the water, your nitrAtes are still 90 at best. Back to back water changes 50% every day or every twelve hours works well, until they are back below 20. (Preferably even lower.) Of course, you won't want to do this rigourous water change schedule, until your nitrItes are under control.