Water changes also bring trace elements that fish need depending on what type of water is being used. I have observed many fish who become more active after water changes. In fact whenever i see any of my fish behaving out of the ordinary I perform a water change whether it has been only a couple days since the last one or about time for one. It is a quick first line of defense. If after twenty four hours I see no improvement I perform my water tests. If they are within acceptable range then I let thefishes immune system do what it can. I sometimes think a water change is like a refreshing shower to the fish though I cannot confirm it.
The most important medication in your fish medicine cabinet is.. Clean water.