Is he already stressed, or are you just trying to prevent it? If he's stressed, I betcha he'll get ich. It's kinda inevitable. If that happens, try slowly lowering the SG to around 1.020 or 1.018. It's low, but it interrupts the reproductive cycle of the protozoan parasite. I did it, and everything was fine. He came back, and is doing great now. If you have a hospital tank big enough for him to swim around, I'd do it in there. Fill it with water from your tank, so he's not further stressed, and then slowly take the salinity down. It worked for me. If you go slow enough, it won't stress him further. If you're just trying to prevent it, then ignore me! :lol:
Sara