Malachite green or formalin won't kill ich. If you go with hypo, your looking at 4 weeks with fish in qt but you still must leave the main tank fishless for at least 6 weeks.
If you go the medicated route, I suggest either Cupramine or CopperSafe, but which ever you use get the appropriate test kit. One is chelated copper sulfate, one non-chelated and each use a specific type of test kit. I use CopperSafe and AP Copper test kit and keep the level between 1.5 - 2 ppm
. The fish will be "clean" 2 weeks after you see the last spot on them if the proper copper level is maintained. The main tank still needs to be fishless for at least 6 weeks again.
If you had other fish in the main, they should also be put into qt.
edited: I can't spell...