Question on water changes/fin rot treatment

[Fenghuang]

One of my male betta's fins are all shredded and the consensus seems to be fin rot. My other betta (the one in my avatar) seems to be fine; however, he always had dark edging on his fins, even when I first got the two of them from the store, so I decided to treat both of them just in case.

As they share a 5 gallon with only a flimsy homemade divider between them, I jarred them both and have them floating in their tank to stay warm. I don't have aquarium salt and I can't go to the store to buy any until next week so all I'm doing is the water changes and trying to promote regrowth with Stress Coat until I can. I am really worried that the daily water changes will damage their fins more or stress them out. Does anyone know what is the best way to perform 100 percent water changes in smaller containers? Any help will be extremely appreciated.

[InfiniteGlory]

I went ahead and purchased a small 1 gallon tank to treat my betta when he had fin rot. I would cup him every time I wanted to do a water change. Smaller tanks are just easier to work with in a situation like this.

Also I have read some posts where simple table salt can be used if you do not have aquarium salt (AS). I believe it has to be the non iodized kind but do a search for salt and see what you can find.