This is a black polycarbonate lid to place over the corner overflow boxes in a reef tank.  This lid fits on the inner lip of the tank's trim, preventing light from filling that area leading to nuisance algae. This should keep fish and other livestock out of the overflow. The long edges measures 8.25".

Made with 1/4" thick black polycarbonate. This material doesn't warp like acrylic will.

Lids are made intentionally larger to both cast a shadow over the teeth gaps of the overflow to avoid nuisance algae growth, as well as to create a handy bumper when floating bags of livestock in the aquarium. The lid holds the bag away from the drain grate so water continues to flow without obstruction.