To answer your questions:
Lowering salinity is not very harmful to fish. So, you can lower it in two days if you wanted to. You didn't lower it too slowly, but it could have been lowered faster.
You're doing something wrong. Quarantine all fish. Don't use medications that only work some of the time. Stick with tried and true methods. You now need to do two things: Get it out of your display and don't let it in again. I have not had marine ich in my display aquariums since 1971. After 600+ fishes I've kept, I think it is a prime example that it
can be done.
It is very easy to contaminate a tank, even when you are close to getting rid of it. Attention to details and patience will get you to a level of expert in no time.
This is some reading for you. Read it; do it!
http://reefkeeping.com/issues/2004-1...ture/index.htm
Steven Pro’s article on Marine Ich (Part 1):
http://reefkeeping.com/issues/2003-08/sp/index.htm
(Part 2):
http://reefkeeping.com/issues/2003-1...ture/index.htm
If you have questions, we'll either point you in the right direction or answer them.
