I've never tried this myself but I know that a few people on here do run "rescues" the one I got a little involved in was sena's, the best thing you can do is make sure to have a full medical kit on hand, lots of clean tanks but also, by pulling out the worst fish in the store and showing them to the employees saying you want to take them home to care for them but you can't pay the designated price because there is a big risk the fish may die in your care, I believe a few people managed to get reduced price bettas nurse them back to health then sell them on to people who know how to care for them. use the proceeds from selling them to buy more meds and make the hospital tanks the best they can be, also another source of income would be adviseable, you could find something you are good at making and sell in order to earn more money to keep the rescue going lol these are all just ideas, its up to you what you do with them