Hey Daz209. You got your betta the way i did, as a gift . Never care about fish before but in love right now. If you fell in love then definitely rescue one from that terrible stores where those beautiful bettas dying.
About bettafix please check the link, check my post #24. And you can also check whole post in general its a lot of tips there
Also for the one gallon and 2.5 gall size tank without the filter , which is absolutely fine do not have a filter for that size of the tank, you need to do 2-50% and 1-100% water changes a week or just one 100% every about 5 days. So i think that also can lead to the problem.
Problem that he had with swimming can be SBD or internal parasites, or internal bacteria ....we don't know since we would need more details. He also might has other external bacterial infection along with internal problems.
Check the link i gave you there is also information how to acclimate the betta to the new water when you doing full 100% water changes, well if you will have new betta...
And when you go to the store try to check the betta for any symptoms of the disease . You don't want betta to be bloated, swim on the side, stay at the bottom, hanging at the top, any visible symptoms on the body like white fluffy patches , long flowing fins,full fins, you don't want to see dark ,discolored edges that are ''smudged'' on the fins.
I would also disinfect the tank and everything in the tank if you decide to buy another betta.
Please let us know if you need any tips on the tank disinfection. I have a very good link to follow.
I know some people scared to buy a betta after they have bad experience , but believe me you only do them a favor rescuing them from the store. And looks like you care since your was doing really good job trying to save him.
Also if you really afraid to buy and get the sick betta from the store let us know and i can give you a few people who breeding them and can mail you healthy betta.