I used to have an open top and then I got my teminik wrasse so I had to cover it. I was using egg crate but it cuts down alot on the light. So today I made new tops out of the bird netting/wild life stuff from the garden section at lowes. Went the hardware section and picked up the window frame kits. You cut the aluminum fram with a hacksaw. Insert your corner pieces and lay down the netting. Then insert the spline material and pull the netting tight as you go. I really like it. 1/4" squares so it's still doing the job of the egg crate but it lets so much more light in. Very easy DIY project.