From what I understand, once a tank has Ich it will never truly go away until you remove all the fish from the tank and leave it for a few weeks so the cysts have nothing to host to and multiply, instead die. The fish who have Ich would have to be quarantined and cured. Freshwater baths, Ich medicine, formalin baths? I can't remember all that is required, you'll have to look into it if you want to erradicate it for good. If not, I imagine whenever the tank is stressed, Ich will pop up on the more susceptible fish. Let me know if you want me to forward on the exact instructions, I have a few books that address this. Or book recommendations.
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