QT help! - Ammonia and Nitrites rising! Amquel+ false positive?

[qfrisco]

Hi all - I've had a Powder Brown Tang (new fish) in my 15g QT for going on a week now. He broke out with ich spots the follow morning, which I was half expecting, given how susceptible these guys are to ich. The ich spots have since subsided, to where I currently don't see any more spots.

I'm treating the ich with the water change method - 50% water changes everyday for two weeks. I prefer this method as I think it's too much of a pain maintaining proper pH in hyposaline water. Anyway, this post isn't about ich treating methods - but rather my rising ammonia and nitrites! I also use a Marineland Power Sponge Filter, and had the sponge sitting in my sump for a couple months.

Even with the 50% water changes daily, I can't get rid of my ammonia and nitrites. My ammonia is now up to 1.0, and nitrites is up to 0.25. I used a dose of Amquel+ when I first filled-up the QT. I know that some ammonia tests can register false positives with Amquel, but do false positives still happen with Amquel+?

I am using the API ammonia (salicylate based) and nitrite tests.

Thanks!

ETA: I am also using RO/DI water, aged at least 24 hours before I do the day's water change. I tested my makeup water and get zero ammonia and nitrites.