Just a thought...Maybe larger water changes would help you out and possibly more filtration. Not sure what kind of hang on you have, but our Penguin 330 was doing a pretty good job til I decided to try growing corals and upped my filtration. My nephew has a lionfish, yellow tang, foxface and humu humu trigger in a 55 and all seem to be doing fine with, I think, 2 P. 330s rolling. Of course, this is a fish only w/ a little LR and I think the bioload is too much, but he does what he does. Anyway, he also has high nitrates and, like I said, the fish seem to be healthy (despite my concern). I can only guess that he does larger changes than myself, at least when I was there, it was more than 10%. Not sure about the frequency.
Is your goal to have a reef w/ corals, inverts and such? If so, be patient and try small equip. additions when you can afford them. If your fish are fine, then I'd say you're doing something right. I could ramble on more if you like, but I'll try getting back to your ?.
As far as AmQuel, I use it with no ill effects. I personally don't have a nitrate problem as far as I've tested, but I may have a diff set up also. I add a little AmQuel to my new batch of water when I make a change and let it brew for at least a day with circulation and temp regulation. I also dose a little Cycle every once in a while to make sure beneficial bacteria is well populated. HTH and good luck
