Balloon Molly problem

[downtown pat]

Balloon molly, about 3 months old, roughly 5/8" long, in 20 gal. tank with 20 'littermates" (no other fish) suddenly has caudal fin cocked upwards, at 45-degree angle to body. Eating well, but tends to hang out at rear of tank, and swims vertically to surface for food. Have never seen this in many years of fishkeeping. I spend more time watching this tank than I care to admit, so I don't think it's a congenital abnormality. Disease? Injury?

 

[BillD]

Balloon mollies are already congenitally abnormal. It wouldn't be a stretch for it to develop another, although it could be an injury.