Parameters are the Nitrates, nitrites, PH, Ammonia, calcium, alk....
If you don't have a kit, you should take a sample of water (like half a water bottle) to your LFS and have them test it for you.
Also, how long has the tank been in your possession? If it was just recently transported to your place, and you're still setting up the rock, the fish could still be stressed. Tangs are very very notorious for having ich, and once they get it, it can and will spread to the rest of the tank if it gets bad enough. The only thing i could say is to scrap the idea for the plate coral, and buy some meds for your tank. Since i'm sure you have/want a reef tank, i would get some natural stuff. My other tank had a bad case of ich, and i used some natural ich meds (Can't remember the name). I did double the dose of what it recommends, and it got rid of my ich case, and i've never had it again.