I'm an experienced fresh water aquarium owner (10 plus years) but am relatively new to betas. About a year ago I got a beta and had it in a bowl as I was unaware they could even go in a community tank! But after a few months I noticed he was getting some fin rot. I figured it was because the water quality was poor and it might be worth it to try to put him into my 42 gallon planted tank. I have two large danios two rainbow plecos two golden rams a khuli loach and a frog. To my surprise it worked great and they all proved to be excellent tank mates as he seemed visibly wayyy happier and the fin rot went away. Then a couple months later I got a juvenile black ghost knife, as the pet store people said it would be a fine tank mate provided i do a size upgrade in the future. And he is, everyone is happy, BUT the beta has had fin rot ever since! The ghost knife spends most of his day hidden in a castle and doesn't seem to bother anyone so I know he isn't nipping. I tried three different types of anti fungals and eventually it got so bad I decided to quaruntine the beta in a floating box. The first time it worked well, his fins healed and I released him back into the tank after a week. But within a day of being out the fin rot came back. I've tried this for a couple months including cleaning the tank excessively (I always keep a very clean tank) and it seems the only thing keeping him alive is being quaruntined in the floating box. The only thing I can think is that it's just stress from over crowding so I'm going to upgrade to a 56 gallon (as I was going to upgrade anyway for the knife even though he's still really small). Was just wondering peoples opinions on how I should do it? Should I put the beta in first and slowly add others? Put them all in at once? Or put the ghost knife in first? Any tips will help! This was once a very happy community fish and I just want to get him back to that.