ATI makes dimmable t5's. Or you could try a led fixture whether it be a diy or store bought
I've noticed this with my Cardinal Tetras. They settle in between the plants before the lights go off at 10:00 PM. The Espei and Hengels Rasboras will congregate at the surface after the lights go off though. Perhaps the ambient light is sufficient for them.I use a relatively cheap system of lights and timers to light /darken in stages. The fish "sense" time in some way and I can see them begin to settle into the sleeping places as the lights begin to go down.
This is on a 125 stocked with Malawi cichlids who do not always get along well during the day. But as night comes closer, they begin to act different. Some come together as a group in one open corner while others back into spaces between plants. One Kenyi rules a corner all day but at night a group of 5-6 juvies gather with him behind "his" rock where they all settle down together.
There seems to be some advantage to letting them do this in a more organized way with a bit of notice.