I would use egg crate depending on the fish size. It will allow more water flow through the holes.
“We didn’t underestimate them. They were just a lot better than we thought.” -Bobby Robson
“Football is a simple game; 22 men chase a ball for 90 minutes and at the end, the Germans win.” -Gary Linekar
"Some people are on the pitch! They think it's all over! It is now, it's four!" -Kenneth Wolstenholme