Piece of branch (or bamboo)
Filtering material (cloth)
Aquarium air pump (any)
Step 1: Take a Piece of Wood
as long, as deep is your aquarium. Drill a hole in diameter approx.12-16 mm along it. Connect outlet of air compressor to lower end by sharp angle to top. Fit it in vertical position in your aquarium Leave about 1-2 cm above surface, enwinded by any filtering material.
Under water it looks like simple wood branch. And if the top of filter is hidden,
it will become even more quiet and invisible.( use smth. like a small cup: peace of wood, or little nut)