Bottom line is the maintank needs to run fallow (fish free) for 3-4 weeks longer if possible. ICH can only survive on fish NOT inverts, so remove the fish. No other options exist, remove all fish and treat all fish with hypo or copper. Copper can harm some species of fish. Like anything else on here, take the advice with a grain of salt. (no pun intended).
Beth is the true disease expert and I would listen to anything she says. Look up ICH and find her threads.