If you're getting a bigger tank it'll depend on the tank size & if you're running a filter. I have a 2.5g unfiltered tank that I do 2 50%+ water changes per week with a 100% every 2 weeks or so. On my 5.5g filtered & cycled tank I do a weekly 50%+ water change, the same for my 10g tanks & 46g tank. IME if you're doing the right amount of regular water changes the fin rot should heal up, if the fin rot is more than mild extra water changes can help. If you plan to run a filter you'll want to read up cycling a tank, sounds complicated but it really isn't. IMO if you buy a tank smaller than 5g I wouldn't run a filter.