I can say for fish, but I used to have a frog (we think it was a bullfrog), that used to hide inside the filter inlet. I don't remember what brand filter, but it may have been a Whisper as well. It was an internal, that much I can tell you with certainty.
The frog gave us quite a scare the first time we could find it anywhere... and then I say a leaf piece on the floor that kind of resembled a webbed foot. I thought for sure the frog had somehow escaped, and dried up or something somehow. I don't know what I did it, but I shook the filter for some reason, and out swam the frog, none the worse for wear.
There is water flow in the filter, so I imagine the fish is getting to stay oxygenated. Perhaps it just likes the feel of fast flowing water.
