I learned the hard way, My fish had ich when I was starting out 4-5 years ago, classic case of new-tank syndrome. I went to the lfs and bought what they recommended but they didn't tell me about the carbon and I guessed I didn't even realize I had carbon in my whisper cartridge until the water kept losing the blue color of the medication. It wasn't something they warned me about either. Glad you asked the question. Good luck with the treatment. By the way, if you don't already know it, turning up the temperature will speed up the ich life-cycle and they'll die sooner. The maximum temperature depends on the type of fish you have.