The standard tests are ammonia, nitrite, nitrate, and pH. Liquid tests are highly recommended, like the API f/w Master kit ~$20 in the US, 100-500 tests.
Nitrates are controlled by water changes (in the absence of plants). Nitrates do not usually spike. They slowly accumulate when the WC's don't keep up with the production. Lots more can be said, but this thread is not the place, I'd think. Better start your own thread if want to continue.
I am not aware of what exact toxin causes the gill curl (other than heredics). I thought if anything, it is ammonia and nitrite, long-term. Surely high nitrates do not help any but I'd think it is just a complicating factor, not the cause.