You are correct. Most fantails like moors are not the best swimmers. However does he float near the surface and if he swims down does he float back up on his own? Due to the selective crossbreeding of goldfish many have swim bladder problems, which cause them to float (positive buoancy). usually it does not affect them greatly unless they struggle so much and can not right themselves or feed effectively. I have read of breeders using hyperdermic needles to correct this by removing air!! I dont think I'd try this as I'd probably end up stabbin him to death. Best bet is to reduce the current. I have a 3 inch calico ryukin with a swim bladder problem. He just works a little harder, although it is somewhat amusing when he swims upside down after gettin stuck in the current. As long as he is healthy and thrives I wouldnt worry about it.