I drilled three 5mm holes in the cover and put 5mm warm white led spots in. On top there is a cut in half along the lenght pvc pipe that I spray painted black. I'll make some pics.
When its dark I can see the fish swimming in and out of the spots.
The main light is on from 10am until 6pm. The LEDs are on from 6pm until 12 and from 6am until 10. So the fish still have 6 hours to rest.
It costs less than 5 dollars, a simple moonlight would have cost about $25