If it's ICH, then other fish could possibly get it. Not 100% if they are resistant to ICH and strong/heathly.
I once had a Copperband show 100% ICH, but not other fish had symptoms. He was so ICH covered he way dying.
I removed the Copperband. Treated in a hospital Tank and saved him.
I forget which method I used, it may have been copper, but many times I "Succssfully" treated fish with ICH with the "
Tank Tansfer method". Works every time, if you have the setup, dicipline and patience to do it.
TTM has become my favorite for Treating VISIBLE ICH.
https://www.reef2reef.com/threads/tank-transfer-method.192655/
I do use Copper if I get suspicious observing a fish during QT.
I learned the hard way that you must keep your copper levels proper (with a copper tester or kit).
Too high Copper Levels or too quick Level increase, can kill sensitive fish in hours.
Too low level,
even briefly and the treatment will not work.
So with the ICH fish in Hospital I worried that the Display Tank the Copperband came from would need to go Fallow, and that would be a pain. I once went fallow after VELVET wiped out all my fish. Never wanted to do that again.
Yet, like you, the other fish showed no ICH.
So instead of treating all the fish, I left them in the Display tank and used PolyLab Medic with the mentality "it can't hurt to try". To this day years later, that tank never got ICH. Copperband after treatment was put back and even new fish since that time. No ICH since that time, and I never went Fallow. So maybe the Polyp Lab did work, or maybe it was something else like the Heathy fish resisted hosting the ICH parasite and that ended the ICH cycle.
So my recommendation, is:
- Get your ICH fish out and treat it in a hospital tank (your preferred method Copper/TTM).
- Observe the other fish while using the Polyp Lab which is invert and reef safe product (just don't drop the granulues directly on corals), and hope for the best. It's not too expensive and may work. Can't hurt.
All the best. Hope your sick fish gets better and best of health to the others.