Yup, the long flowy fins make it hard for bettas to swim against the current for more than a short burst. I had one for a while in my 30 gallon, and he was doing fine for a while. Suffered some late night aggression that he didn't survive though. That's one of the other things, other than bettas themselves being very agressive towards anything that looks like another betta, any fish than has a tendency towards fin nipping won't resist beating up a betta.