Best to use just tap water, bottled water and such lacks the proper minerals - pH isn't a worry with bettas, they will adjust to whatever is given to them.
Careful using Bettafix - make sure to water it down greatly - not being careful, overdosing, not doing the proper treatment can lead to them suffocating (an ingredient in there easily coats the labyrinth organ and can suffocate them).
If he became better right after the water change, then I would have to assume it was the water change that helped him... something could of been in the water that was causing his health problems.
What size tank is he in, how often do you do water changes, conditioner, temp of tank?
Since he is doing better I would remove him from the medication, best not to risk his organs now that he is acting more normal.
Why I always recommend people to do research before listening to pet store employees - as they may know general knowledge/care, but not everything works for every fish. Discus need specific pH, not bettas. Bottled water is not healthy for a fish. There are many other, healthier alternatives to medication than Bettafix, etc.
I would see if you can find a reason he went downhill - and then make sure to correct any problem. Glad he is doing better.