Raising the water is always a good idea to help the rate at which the ich disease will last, you may also want depending on how many fish are affected, im guessing depending on the time all of them may be because it is exposed throughout the water. What you also should consider is going to your local lfs and get some anti-parasite medication. I have used Mardel's coppersafe which seems to work out pretty well. All i am saying is that treating the water may not be good enough. Always treat the cause, instead of individual fish because you may be losing time.
HTH