Ummm, just a thought.... now I have not seen MY Lion do that.. but I have seen him do plenty of other crazy things..like tonights playing opossum after eating..heheh
But I have a puffer that does just what you are describing.. And it SEEMS like maybe he is sleeping when he does it.. He curls up as much as a puffer can curl. Tail all the way to his face and just floats around like that where the current takes him. Somtimes he wakes up and wont uncurl. He will just kinda flap his fins to get him wherever it is he thinks he is going. But he is perfectly healthy as I've seen other puffers do the same. So maybe your lion is doing this? If he is eating ok and no signs of disease then this is probably the case.... Interesting, that my puffer that does this also eats nothing but shrimp. Maybe it causes a drowsy effect. lol Your lions arent eating any fish at all???