It's tricky to narrow it down without knowledge on the parameters, but atleast once you have the betta you can sort him out. I'd definetly say to do atleast 75% WC and vacuum the moment you get him home, then start with the salt and daily WC. Try and pick up a test kit asoon as you can though just to know what the tanks readings are. Sure you'll sort him out though, he just needs abit of love and TLCI can check them once I have the Betta but she doesn't have a test kit and can't get to a pet store to have it checked right now.