I put these black egg crate lighting diffusers over my indoor pond to be safe. They are easy to lift off, see through, clean, etc. My rays come 5" to 6" up the sides on a regular basis.
http://www.eplastics.com/Plastic/Lighting_Diffusers/B-EGG-375X2X4
http://www.eplastics.com/Plastic/Lighting_Diffusers/B-EGG-375X2X4