I have raised this betta from a baby and at an early age developed this whitish mark on his back . 7 months now. I always watch it closely and it always is the same. I think the cause is from his crazy dance he does right before I fed him. But lt has gotten worse. I have read alot of information on betta disease but this one has really stumped me. Acts normal swimming around his tank eats fine. His tank mate has fin rot right now so presently I have them in separate tanks and treating them both with aquarium salt and microbe-lift Artemis. I have a bunch of tanks this one is a 6 gallon tank and I regularly clean tank. weekly vacuums as well as a quick vacuum mid week, yes cycled, water tests are all good.