Need Advice: How to cover the bottom of your tank

[Coleus]

Well, my tank has no sand or substrate so refugium light gets into my tank at night which bothers some of living creatures in my tank.

What is the best way of cover this? Since it is drilled and bowfrontso it just so hard to complete cover it with black film background, I tried and could not get it to block all the light.

I am thinking of duct tape but then, light still leaks through because the tape does not block out the light.

Any other solutions beside painting it because i really don't want the paint potential leak into my tank water and will kill everything.

Thanks