I have an enclosed canopy on 3 sides and have had several fishf jump out. I even had a oscellaris clown jump through a cooling fan (it was off at the time). It just depends on what you have in your tank. Some fish are notorious jumpers such as hawkfish and some wrasses. My next hood is going to be open on the back but have some sort of screen on the back. I call it a fish trampoline. My other hood is completely enclosed and guess what? I have never had a fish jump out of it.
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano