with my small tank the more shy fish come out.and there are alot more fights!
with the bigger tank all of the fish seem to slow down,and some rest on the bottom.and the big catfish come out!
im not sure fish sleep though.
Fish don't actually sleep, but many diurnal fish do manage a sort of comatose state (while some never do stop moving, even at night). Nocturnal fish are obviously active at night.
My large tank is a hub bub of activity at night. My fire eel comes out to play with my plec and featherfin catfish. In the morning there's often a change of scenery where they've moved things around
My smaller tank is much quieter - everyone's quite still hiding in the plants, trying to keep out of the way of the chaca chaca