I have tried this stuff before in my previous tank. After 1 day's does I had some LPS and sps that looked really bad and didn't recover in the end. I have also tried Rx-P, and have had good results with it if you catch the ich very early and before it has spread to other fish. From what I've read, basically it irritates the fish's skin a little so the fish thickens it's slime coat...making it more difficult for the ich parasite to attach/burrow. You need to be very diligent with dosing though, and I have used it for 5 weeks straight (longer than stated on the bottle) to ensure a full ich life cycle. Don't stop dosing once you don't see white spots anymore! This doesn't actually kill the parasite, so if the white spots are gone, there could still be ich in the free swimming life stage in the water. That's why dosing for such an extended period of time is important. I haven't had any ill effects on coral or inverts with this stuff though I haven't tested it in an SPS evironment so I can only say this for softies and LPS with any confidence.
Of course the best route is always to quarantine fish first, but since this one is already in the tank and I'm sure impossible to get out, I would suggest giving Rx-p a try.
