We use foam to cover all pump, skimmer and powerhead intakes. Even our hatchlings don't go through them and we have never had a blue-ring, A. aculeatus, or other species sucked into a pump. One O. cyanea did pull of a foam cap on a powerhead, but it was too large to enter the mechanism. The worst that happened was that a couple of arm tips were inserted and were chewed uup.
Roy