I have a manual for Fluval 104, 204, 304 & 404. It says;
Place the filter in desired location beside or beneath the aquarium. Since the filter uses gravity feed, ensure the filter is stationed below the water level of the aquarium. The it has an outlined box with these words in bold Filter must never be installed above water level.
My Jebo 819 manual says "It must use below the water surface of the tank".
It alo says "Move the machine by holding its bottom, not only the ouches, other wise the ouches and the machine body will be broken, water will enter and cause accident" :lol:
And then there's the "In case of accident, pull off the power plugs of this machine and other electrical appliances which may also cause leakage of electricity and fire."