Morepower from what I have learned in my experience is you need two tanks in this hobby.
A QT and a DT.
Combating ICH is to simply eradicate it before it hits your display. QT everything using cupramine and prazipro.
Don't ever put anything in your DT unless you medicate it first with the meds I mentioned above.
I have wasted and killed more fish then a lot of folks on here because I refused to QT. Finally after realizing and loosing ALOT of money, I have truly seen where QT with cupramine and prazipro works!!!
I have 4 tangs that have received this above treatment, an achilles tang, hippo tang, powder blue tang and a sailfin tang. All shared space in a 90 gallon tank which is small however they are not large by any means.
They fight a little from time to time but nothing too severe. Is there tension and stress in the tank? Of course, but do you know after a few weeks none of these fish have a single white spot, no flashing and they are healthy specimens. My fish most likely are stressed a little due to QT tank size and still not a single white spot
Personally I think feeding with Nori and Garlic simply keeps the fish immune system in the best possible condition. The fish however will still have ich and will be suffering. ICH population in your tank will continue to get worse. Some fish might be able to fight if off to where you don't see the fish covered with cysts but they will still have it. If you see a fish flashing against PVC pipes or rocks most likely he has ICH.
Eradicate the parasite and you don't have to worry!
After seeing the outcome of my tangs health, your crazy not to QT/medicate all fish. Hell I might QT my hand before I reach in the DT for any reason
buy a few 10 gallon tanks or use some plastic rubber maids you might have around the house.
You will have to do this if you care for your pets.
You will need the meds I mentioned above plus ammonia tester and a seachem badge for each tank.
Treat for 4 weeks and let your DT sit fallow a minimum of 8 weeks. It's a painful process but you will be glad that you did it in the long run...
I have used every magic ICH formula known to man. The only thing that I have found to work is using cupramine and prazipro to fight against ICH and flukes.
I'll say again, it works!
Good luck....