Once one fish shows symptoms of ich your entire tank has been exposed. You should take them out of QT, put them back in their tank and treat the entire tank for ich.
I've used Kordon's Rid-Ich with great success.
This article may be of interest:
The Skeptical Aquarist
PS. Each of your tanks houses fish that are sensitive to meds so you will need to use the meds at 1/2 the required dose.
Sharks, Cats & Loaches are considered "sensitive" fish due to their lack of scales.