It probably depends on the power head and how small or large the opening is where water gets drawn into.
Some powerhead have a built in (or optional) sponge sleeve or fine floss cylindrical cartridge deal that fits on the intake of the powerhead.
This would protect small fish from being sucked in.
Some people attach a water bottle to the intake with the end cut off, and something like Polyfil or a layer of foam sponge is placed inside.
Sort of a DIY water polisher that also protects fish from having a close encounter with the impeller.