If it's silvery with black tips, it's a bala shark, not catfish. They do have scales so heat+salt will work well. Turn up the temp to 86F+, that will halt the lifecycle of the ich parasite, and add salt to kill of the existing ich. I'm not sure exactly how much salt, but I'm sure someone will come along and tell you. Just do that for 2 weeks and you should be good.
How far along are you with the ich guard treatment. It might not be a good idea to have both meds and salt. And like bluerose said, by the time you noticed ich on the shark, it probably spread in the main tank so you should use the salt treatment on your main as well.
--Adeeb