I think that ick doesn't stay in the tank after a successful treatment. Last year, I had a long stressful bout with ick in my large and fully planted tank. I spent about 3 months trying out almost every medication (Ick Attack, Ick+, RidIck, Copper Safe, Ick Away, etc.) there is except for salt since salt kills plants. I even netted out fish with ick and put them in a hospital tank, but then other fish in the main tank started to have ick too. I ended up wasting most of my plants and most of my fish with either medication or with ick, not to mention the stress and the daily labor of changing water. The whole ordeal cost me hundreds of $ for dead fish, dead plants, and medication.
I finally use the salt-heat (gradually adding salt to about 4.5 Tpsp/gal and 86F) treatment in the hospital tank, bare glass tank with just foam fitration and live food once a day only. This works with every fish (even with delicate clown loaches, cardinal tetras, ...). The key is making the changes slowly, 1 Tbsp and 2 degree F per 12 hours or so. This finally cured ick totally but very slowly, about 3 weeks or so. However, I couldn't use salt in the planted tank and couldn't put all the rest of the fish in the hospital tank.
Finally, I bought a diatom filter (Vortex) from thatpetplace.com for about $70. Boy! This worked wonder. Running this 24/7 for about a week totally eradicate the ick in my main tank without any other treatment. The filter got clogged every 3 days or so and had to be recharged. I just got the diatom powder from the home center ($10 for 25lbs vs. $5 for 1 lb from the pet supplies), just make sure that the powder doesn't have added chlorine. Since then, ick hasn't come back. I still have all the ick medication and salt that I may never use again and I have had ick for months since.
Now, I put newly purchased fish in the quarantine tank and run the diatom filter for about 3-4 days before putting them in the main tank. The diatom filter hasn't been put to work much now that everything is fine.
I now $70 sounds like a big investment, but if I had known what I know now, I would have bought this the first day that my fish got ick. It would have saved my all the money on dead fish, dead plants, medication, and all the stress and laborous daily water changes

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pkn