May be a bad pic (i.e you caught it at a bad angle), but usually when they are healthy their fins are erect unless they are in a swimming motion (not hovering about, but moving left/right, sorry i've explained that bad, sure you know what i mean). Its rear fin seems curved up slightly, but doesnt seem to be 'deformed' as such.
As long as its swimming around ok, and seems healthy, i'm sure there is nothing to worry about.