If not light when do fish lean?

[stardotstar]

I have a silver shark that is leaning to the right.

I know they lean when the light/sun strikes the tank at odd angles but why else?

Will it recover? It is in every other respect a very healthy looking specimen. Should it be separated?

Will

[William]

Have it been leaning long?
Any other fish showing strange behaviours?
How is the water quality?