I do not think you'll get any negative feedback from me or others about these, many Killifish folks use similar things.

Foam is cheap and DIY works well.

But with planted tanks, the plants do much more than these filters when properly grown correctly.

They also look much nicer when that is done also
These foam block filters are as old as the hills, but they can get covered with algae, moss and need cleaned in a planted tank every so often.

As they accumulate detritus, they will sour and the flow reduces and the cycling is no longer the same as it once was. O2 levels decline, then you get algae.

My point is that you will have better results if you clean these things.

My old idea used open cell foam I used for produce and meat counters under the rack, Vestax is what the brand name was, about 1/2" thick and black in color.

I would cut to size and then roll it up to whatever thickness I desired like a burrito.
I'd add two large Broccoli rubber bands to fold the rolled up sponge filter in place.

Cost: free

I'd use a pvc pipe with holes and make an adaptor to fit on the power head and cap the other end.

Same type of idea, but uses less space.
I can put up some math and make the idea seem legitimate and well thought out, lending credibility to an idea, but the idea is so old already. I prefer sumps and canisters for a few simple reasons: they take up the least amount of space and look good.

I can afford a nice filter and lighting, so I try and make the tanks I have look nice inside and out. For a lot of folks, they like the DIY stuff, but making the DIY look good is the next goal.

You may use cork bark to hide the sponge and prevent it from getting algae
This is also a good spot to fed the CO2 into the powerhead as the cork will hide the CO2 line(heater etc).

Add some moss, java fern, Bolbitus, Anubias, etc to the cork, along the top, add some pennywort.



Regards,
Tom Barr