For that bioload you will need a 40 breeder at the very least. Or get several 20G tanks and break them up. Get a HOB Aquaclear filter 70 and only run mechanical filtration in it. No carbon as it will pull the copper out. Put a basic light over the tank and a heater. Place a bunch of sections of PVC in there for hiding places and be sure to get a SeaChem Ammonia Alert tag and be ready to do daily water changes on it.
Hyposalinity will not kill ICH. Copper is the only thing that will and it has to be within .2-.5 ppm. If you have a fish that is sensitive to copper then it will need to be in it's own tank.
If it's a fish only tank, then removing the inverts is a must. Otherwise you will need to catch all of the fish and move them to a hospital tank or tanks for treatment. If you do not have any ready to go and need something immediate go get some of these.
https://www.homedepot.com/p/HDX-27-Gal-Tough-Storage-Tote-in-Black-HDX27GONLINE-5/205978361
They are reef safe and you can put all your fish in there with a heater/ HOB filter or powerhead and start treatment. The main tank will need to remain fishless for at least 72 days to ensure the ICH parasite is completely dead.
If you don't want to go that route and do not have corals, pull the inverts and treat the tank with Cupramine as it stays in suspension and won't leech into the rock like standard copper. You will need to do treat for at least a month until there are no signs of infection. Then use Cuprasorb to pull the meds out and then do multiple large water changes and test for copper after each one until you get at least 3-4 successive 0 copper results.
Good luck.