Well, it kind of pushes water out of those slits in the side rather than sucking it in, so it's extremely rare that a fish will get chopped up by it. BUT, Hydor does sell a thin foam cover that you can use to cover up those slits. Never had to use them though, never had a problem with fish getting chopped up by them, and honestly, i've reviewed many many tanks all using Koralia powerheads, and i've never heard of fish getting chopped up by them.
However. I will state that if you turn the powerheads off to feed the tank, or if there is a power outage, you MUST look inside of them before you turn them back on. Because i have heard of people turning off their powerheads, being careless and turning them back on, and unknowingly having a fish get chopped up because the fish went in the powerhead after it was turned off.