I have a big GPASI auction I'm going to on Sunday and have people bringing me some fish and hoping to get some rare fish as well. Problem is I'm setting up tanks for them using cycled sponge filters but both the filters are in my 75. Will the ich contaminate the sponge filters or does it just affect the fish in that tank? I'd like to be able to get the fish otherwise somebody else will just buy them. This happened at the worst possible time.
Ich has free swimming stages so can be in any water (or item in the water.) There is also a spore stage where it tends to reside in the substrate and a cyst stage where it's attached to a host. I think you can go with your plans and just assume the new fish already have ich. It's not a bad quarantine policy anyway.
Obviously do not disinfect the sponges, so just use them as you planned and assume the new fish are sick.