Temperature change does not cause ich. Ich is a parasite that was introduced into your system by a host fish. Your fish have basically been "living with" ich and the stress caused by the temp change, may have reduced your fishes' immune system enough to allow ich to become more prolific.
I'm afraid unless you deal adequately with this problem now, then you will always have ich in your tank. If you just can not QT the fish and treat them, then try heavy garlic soaked food feedings using fresh minced garlic.
Alternatively, its a hassle, but you could remove the LR into a large rubbermaid temporarily and place a PH or 2 in the rubbermaid [depending on how much LR you have] then treat the tank using hyposalinity. If you have any inverts, they will also need to go into the rubbermaid with the LR.
Take a look at the post on ich in the FAQ Thread and you will get info on ich, how the parasite works, and how to effectively treat it.