I had this problem in my FW
but never in my SW
knock on wood please, but what I have done in the past to help some of my SW
friends is to do the following:
move the temp a bit higher than normal 82-84
make the water salinity higher by a bit I cant quite remember the specs
double up on ick medication as follow:
buy ICH attack Organic made from organic matter, this one is a beauty because you can go crazy with it and it wont kill anything besides the ick
at the same as this is going on add the blue iodine treatment I cant quite remember the name but it is made by API if i am not mistaken it is the ICK super blue iodine, be careful if you have crabs/snails/starfish they will suffer from the blue iodine.
by doing this I have seen my friends tanks cure a lot faster than other suggestions, I picked this up on a local trip to the SeaWorld San Antonio, so its no BS.