I’ve noticed they like hiding when I’m in the room. If I sit on my couch so they can’t see me they start zipping around the tank. I’m not sure if they are nocturnal.
Definitely not nocturnal though they do hide from sudden movements. Mine come out when they see me usually because I'm associated with food from above.
If you have a proper set up with hiding places and open room for swimming, they'll be less prone to hiding than they would be with inadequate housing.