My Sumatran and moss barbs are only on the bottom?

Hello, I have a question. I have a 60 liter tank with Sumatra and moss barbs. I have the problem that my fish always stay on the bottom as soon as the light comes on. The timer means the light stays on for 7 hours (3 p.m. – 10 p.m.).

Water values ​​are fine. I do regular water changes. The fish eat and behave normally and swim on top, but only as long as the light is turned off.

I have already dimmed the light using every method possible.

But as I said, as soon as the light comes on the fish are on the bottom.

Maybe someone has a solution

Regards, Matthias. Thanks in advance!