I make mine out of PVC, most of the time if they don't work, it's because there is not enough agitation at the air/water interface, ie enough pumpage.
They remove nitrate precursors, free floating algae, small parasites, and DOC.
Mine is also a biological filter, the stack is filled with lava rock, as biological media.
The pic is from my pond in early spring, tons of stuff to remove after the freeze, and sitting all winter.
They don't always produce billows of foam, because after they have scrubbed the water clean, there isn't enough to remove, and they "rest" until needed.
video is a smaller one, on my 55gal tank, using a small pond as a sump.
