I have used garden shade cloth hung from a hook over the center of the pool. Velco holds it to the inside edge of the pool and the drape prevents any fish from getting caught in a fold, the mesh is small and firm enough to keep most fin types from snagging, and is mildew resitant.. It is pretty easy to arrange it so that the mesh can be raised for viewing and access, it can even be joined to a plastic or stainless frame that can be pulled up with a single cord.