The only proper way to fix this problem would be to take every fish out for 11 weeks (76 days is the longest known period where Ich survived without a host). Then you would have to treat the fish. In my opinion the best and easiest way is to use Cupramine in a QT. This is better than older forms of copper and I successfully treated several fish with Ich using it, and they are still alive several years later.
I used to gamble on not QT'ing before adding to my display and it worked for five years...until it didn't. Trust me, the hassle of removing all of your fish is much worse than proper QT and prophylactic treatment. I've never had Ich since.
Anyone that says Ich is present in every tank is flat out wrong. Nobody has said so thus far in this thread, but it's inevitable that someone will. And again, they are flat out wrong. There's a multi-billion dollar aquaculture industry outside of the hobby that has funded lots of scientific studies on this topic. Fish are the host. No host for the parasite = eventually the parasite dies.
I disagree with the method of "managing the ich" for a few reasons:
1) I don't believe that it's ok to let a living creature intentionally suffer for your convenience.
2) Many of the fish are going to die without treatment in a short time even if some live longer.
3) I don't believe "management" works. Many people will have fish live for several months and call this a success. These people have fish that die after a year or so and call this successful when these fish could live 10-20 years. We all lose fish for one reason or another, but only when you've successfully kept fish for five years or more can you be considered successful, in my view.
4) You can never introduce another fish into your tank without knowingly, intentionally infecting it.