Really it depends on what type of fish you're keeping, some fish are notorious jumpers (ie. firefish, some wrasses, anthias). Others are fine. It all depends on whether you want to risk it or not. The stuff you're talking about is called eggcrate. It is helpful to keep fish in but for smaller fish, sometimes they keep at it until they make it through that hole. Believe me, sometimes, if there is a hole to get through, fish will find it. They're smrt that way
