It took me a few years to learn to protect water intakes of powerheads and pumps. Now I would not think of setting up a tank without it. We use foam and have never had a problem, eve with very small juveniles.
However, I couple of days ago I had an experience that convinced by just how difficult it is to completely protect a tank. We are rearing O. mercatoris and the juveniles are housed on to a tank. Each animal is about 1.5 cm mantle length. These guys are completely nocturnal, so I only ever see them if I come into the lab at night. One animal went missing about two weeks ago and I assumed it died and rotted inside a cavity. I tore the system appart and broke up the LR. No octpus, so I decided to use the tank for some hatching Eupremna. As luck would have it, as I was cleaning off some algae, I knocked off the spray bar. It had some gunk in it so I grabbed a brush to clean it out. There was a white mass inside so I tapped the tube a few times. It would not come out, so I inserted the brush. There was a color change and an arm came out one of the holes. The octopus must have been living in the spray arm for at least a week. Given that it could not have gone through the canister filter and impeller, it had to crawl into the tube through one of the spray holes. I measured them and the diameter is 1.7 mm. I've had octopus squeeze into some pretty tight openings, but I would have thought that there was no way it could or would want to crawl through one of these holes. I guess the morale is to secure both your water intake and output openings.