I'd say the only important, in fact vital chemical, that most of us use is dechlorinator. And you don't even need that if you run off well water or have the means to age/store your municipal water.
"Altering" your water with products is not a good idea. You should work with what you have. The vast array of medications available are also not needed if you run a tight ship.
When I lived in Milwaukee where Chloramine was the primary disinfectant, I used Sodium Thiosulfate to neutralize the chlorine part of the ion, because aging doesn't work on chloramine.
And I am in total agreement with the above, in that choosing the fish to match the water instead of fighting it or chasing parameters is the best way to go
As far as meds my first choice is always just plain salt (not aquarium salt), just plain salt (NaCl),
If its something more serious like a parasite or recognizable disease, that I can determine what that is, I will use that.
Just lately a couple wild fish I caught were obviously infected with Lernaea.

You can see a couple hanging on the lower jaw, and although the larval stage can be eliminated with 3.5 ppt salinity.
The adults are unaffected.
So actual effective meds were used.


Here in Panama, I go to a vet, to buy proper treatment.