If you've got ich severe enough that it's killing fish, garlic and other less drastic methods probably won't work very well, and almost certainly not in time to save afflicted fish. Get the worst fish out and into a hyposalinity OR copper OR formalin/malachite green treatment ASAP. (What treatment is best depends on the kind of fish; some are more sensitive than others to any of those treatments. When in doubt hyposalinity's probably the easiest, but I have heard of some strains of ich that're resistant to this treatment.) Copper'll work for sure, but some fish have real problems with it.
As to getting all of the fish in the tank out ... the best thing you could probably do is drain out most of your tank water (slowly, and without scaring the fish so they don't wedge themselves in the LR,) and catch them in the reduced water volume.
In any case, it's really going to suck.

However, once you do get all of your fish through this and let the tank lay fallow, you shouldn't ever have any problems with ich again -- so long as you follow strict quarantine and cross-contamination-prevention procedures. When you've got a small tank, you can be kinda slack, 'cause it's relatively 'easy' to break it down and fish things out. But it sounds like you've got a significnatly larger system.