I've had this problem with my altuvei and my rhom as well. Mine are around 4-5 inches total length and are still pretty shy. I've had my altuvei for awhile and he's doing better with his shyness that he used to but he still likes to hide sometimes when I turn out the light. As for being nocturnal I have my suspicions at least about my altuvei as he seems to be very active at night. I'll wake up and he'll be swimming near my side of the tank looking at me. I wonder if he even sleeps at all.