They aren't swimming the length of the tank because it's not even long enough to swim up to any normal speed they'd be at int he wild... I have strong continuous flow on mainly one side of my 6ft 150gal tank and they love darting into the flow and fighting it then swimming the length of the tank. Mostly my Achilles anyway. My hippo is too big now but used to do the same. Now he really needs a larger tank. I definitely think they benefit greatly from it. Is something going on with your hippo or is it healing? Its gut looks an odd shape and that facial mark
The facial mark is erosion on just that side, other side is clean. The belly changes, picture was taken in the morning prior to feeding. He is most dominant and first to eat. All these fish are 13 years old.


