No idea if window screen itself is safe or not. I would imagine it would have to be framed, and I have to imagine the frames would rust, or are coated to prevent rust, and such a coating may not be fish friendly. most people ive seen, myself included, use egg crate since its rigid, relatively cheap and easy to work with. The main downfalls are that its kinda ugly and you have to be sure to remove all sharp edges first.