aww i feel bad ..it very nice of you. I used to think like that and try to buy sick bettas. It really hurts to see them in those little cups. But you don't have any betta, i really suggest to buy healthy one and enjoy it. Get experience to take care of it and then if you want more you can rescue one. Unfortunately there is a lot of chance to get sick betta anyway....or see it this way- you will rescue sick betta and the health one that will stay on the shelf will get sick if no one buy it. So really try to get healthy one and enjoy it. You can't change the business with rescuing all the time. And one day you will tired from taking care of sick betta and refuse to buy them :(
A lot of people recommending to disinfect with 10%bleach solution. But in your situation, since external parasites dies without host you can just disinfect with 50% vinegar and water. Wash it very well with hot,warm,cold water. Wipe it and let it sit in the sun for a few days to dry out. Wash it again.
What did you have in the tank?
Sorry i am just wondering how long did you have that betta?
Just in case i suggest to buy aquarium salt at the pet store and epsom salt in any phyrmacy.
What kind of the water conditioner did you use?