I have 2 snowflakes in my reef. They relocate snails and hermits but havent eaten them. Kinda cool to watch LOL There is also a cleaner wrasse which is smaller than the silversides i feed it and they leave it alone. Also 2 small clown fish that they could eat but dont touch. They have very poor eyesight and i feed them a silverside or 2 every 2 to 3 days. Cover every inch of your tank because they will escape!!
I got creative with gutter guard! Also egg crate works well tooI wouldnt recomend plexi glass as the heat will at the least distort it. I try to seal up the whole tank as much as possible. If your canopy is closed on 3 sides just seal up the back.