First-I would QT any new fish before adding to the display tank for at least 2 weeks, 6 is better.......
When fish are stressed they tend to come down with everything and it is so much easier to treat in QT without risking all the fish-and then you have some fish that are more resistant than others too.
I treat ICH by following the parasite life cycle-depending on what tank, fish and how bad it is-I will either turn up the heat (86F), sometimes I use aquarium salt if the fish are salt tolerant, (most times I don't use salt), the big thing is vacuuming the substrate to remove the parasite manually. I sometimes use a two cup method when it is just a mild case or just a couple of fish that are infected.
Since I don't use any OTC medication I can't comment on the product you are using, however, I would start a daily substrate vacuuming regardless to manually remove any parasites.