Remember, 10 days is best.
The high heat is to speed up the life cycle of the parasites.. AS can help but heat is mandatory to speedy recovery. Once the parasites are off your betta, you'll need the highest possible amount of AS in your tank to kill it. Let the treatment continue until deadline. He should be fine then. :)
If you get out 50% of the tank water, put some new water in and then just swish the media in some used tank water. It'll clean it a little bit. You shouldn't clean the tank with anything. Just let it be, the parasites need a host so they'll die in time. You can change the substrate without doing a 100% change, just look into YouTube.
What's the main tank's tank size..?