depends on the type. some fishes are Diurnal and some are Nocturnal.
of course diurnal fishes are active during the light hours and nocturnal fishes rest during that time and are active during the night hours.
i dont believe their are any or many fresh water fishes that sleep, most fishes tend to rest as if they stop moving and pushing water past their gills they would die. When they do rest, most diurnal fishes will be very pale and have their eyes expand inorder to see any movement as possible.
usually with diurnal fish, they will rest when the light has be off for awhile and the surounding environment is totally dark.
example fish: neons,cardinals,tetras etc...
with nocturnal fishes, if u have any in a community tank, they will find a dark area and often rest and when other fishes rest, they will come out and feed.
example: knife fishes, loaches