This depends...is his fins infected or not...if not, I would move him to a different section of the divided tank so that the filter intake or overflow is not in his section and make the water changes I described above
If his fins are infected I would QT and start aquarium salt 1tsp/gal along with 100% daily water changes for 10 days...if you have any IAL or naturally dried native oak leaf I would use them as well for the tannins-I like to steep them in my pre-mixed salt water until the water is a nice amber color-by pre-mixing your salt 1tsp/gal in a 1gal jug this will make correct dosage and water changes easier too.
I like to maintain a water temp in the 76-78F range when I treat.
If you Qt-I would still make some extra water only changes on the 10gal- especially if his fins are infected-this will decrease the bad bacteria numbers in the 10gal to prevent any issues with the other two and be careful with the water temp...you really want to try and keep it under 80F to prevent other health related problems from high bacteria-they love warm water.......
I would also give my filter media a rinse/swish in old tank water with a water change to maintain the water flow and the good bacteria.