"Treating" generally means the fish are sick, in my book.
I'm sure some people add salt an tea leaves/melafix on a regular basis.
However, if your fish are doing well and you aren't using distilled water, your fish should generally be ok. However, some fish like mollies and other brackish fish require a decent amount of salt, whereas fish like rainbows don't really show their best colors unless the tank water is less than clear (i.e. with tannins from bogwood).
a water conditioner that deals with chlorine, chloramine, and heavy metals is all I ever regularly put into my water. That is all unless your fish need something specific, which is why you should research any fish thoroughly before purchase. But, in general, freshwater fish don't need anything besides dechlorinated water.
yeah i was thinking that. just when you go in shops you see loads of the water treatment stuff and i have never bothered and have had the tank nearly a year and my other tank over a year. they all seem ok. i do get an odd fish every so often but thats it really.