I'm glad my skimmer has a solid sponge filter that the water sucks through before the pump. I've rescued Aris the red slider turtle from there many times, as well as goldfish and frogs. Never have had a koi get in there, but then my koi are larger than the goldies. The turtle was a slow learner, LOL, but the goldfish never got in there twice.
My pump that runs the waterfall and fountain was on the bottom, but then put it up about 5" on a rock so that the sludge from the bottom would not constantly clog it up. Instead, I use a net and net out the bottom grudge, as it comes out in a solid matt, all gathers very nicely in a few sections, due to the Pond Perfect I use in the summertime. It helps matt old plant matter, fish waste, etc. together for easier removal. Also, with this product, if I had the pump on the bottom, it would get clogged even faster, as whole matts of gunk would get caught on the screen.