What others have said is true. The medication you are using is not effective against ich. It may say it is, but it doesn't work 100%. If it is invert safe, it will not kill all the ich. And the only way you can rid your main tank of ich is by removing ALL your fish from it, and leaving it empty of fish for several months. In the meantime, all your fish that you've removed need to be quarantined and treated using a method that DOES kill ich, such as copper or hyposalinity.
That sounds like a hassle, and it is. Your other option is to live with having ich in your tank (which is what most of us do) and try to keep your fish healthy enough so that they don't come down with it. If you minimize stress and keep your fish eating, they probably won't ever have problems with ich.
If you buy really difficult and sickly fish like clown tangs, well that changes things :) Those fish are really susceptible to any and all diseases out there, and will probably get sick just from you looking at them funny.
Stick with tough fish, and you should be okay.