Betta Fish: Bought a sick betta

Hi, I went to PetSmart with the intention of getting plants and I saw this adorable little guy that didn’t appear to be in the best conditions. He keeps floating on his side. I have multiple fully cycled tanks to add him to but I’m not sure exactly what’s wrong with him and how I should treat him. Should I isolate him in a separate hospital tank? I have a 13 gallon long tank with eight glow light tetras I can put him in but I don’t want my other fish getting sick if it’s something bacterial.