Most sources will tell you that Ich is always present in the water, it can only cause an outbreak on fish if the fishes immune system is weakened or outside factors cause stress which leaves fish vulnerable.
Not all medication will tell you to change 25% of the water, but doing water changes is a good start for any disease as it will help dilute pollutants, remove organic waste and is pretty much like opening a window in a stuffy room.
The best treatment for Ich in my experience is prevention through cleanliness and avoiding unnecessary stress on the fish, if Ich does surface then I usually treat with high temoperatures (29C for five days initially) and perhaps meds if it is a particularly nasty case. I only use King British WS3 or tetra contra spot, and always use an airstone when using medications with formalin.
HTH
Ken