[Albtraum]

I know these can be bought pre-made but I want a custom size. Trying to take a big 4" x 4" x 12" rectangle foam filter media piece to a cylinder shape like in the picture, which slips over the water pumps inlet.

*photo credit to PL user burr740*

I want it smaller than pictured, maybe 2" diameter or less, then about 3 1/2" long.... but how do I cut a clean smooth circle the whole length of the cylinder?

I'm thinking razor blade and squish it down somehow?

 

[roadmaster]

I might look at Aquaclear quick filter attachment which comes with cylindrical foam insert.
I use these with the aquaclear 70 powerhead and bought extra foam insert's to use as prefilter's for filter intakes,and to swap out dirty insert for clean ones at weekly water change.
Not very expensive for the foam insert's.(can cut easily with sharp scissors)
Fluval offer's a prefilter also but the density is too fine and clog's quickly .

 

[theatermusic87]

Fluval has one that's a smaller prefilter that's about the dimensions you're looking for and also very inexpensive
https://www.amazon.com/Fluval-A1387-EDGE-Pre-Filter-Sponge/dp/B002LL32RY

If you're looking to do it yourself, make sure you have open cell foam so water can move through it, and if you get the foam damp then freeze it, it becomes much easier to work with tools and a hole saw and drill will likely do the trick

 

[roadmaster]

Hmm,
Just tried to google Aquaclear quick filter attachment and all that is shown is the polishing insert which you don't want for the purpose stated.
Mine came with the black foam insert and I might have bought the replacement's at BigAl's online (can't remember).
PM me with address, and I will send you a spare I have lying around (no charge),
if you can't make or find something else.
I even have a couple of the fluval prefilter's you could try but the foam was not porous enough for my use.(Petco).

 

[PlantedRich]

Some reason it requires round?
Sounds like it's making a simple move difficult. I like the AC sponge for the right pore size and cut a slot with a knife so it will slip over the intake.

 

[Albtraum]

Well I ordered that Fluval dense foam sponge theatermusic87 suggested, for another tank that might have fry in the future. Gonna try freezing it and cutting it in a bit. Thanks for the suggestions everyone. It's definitely large open cell, razor blade is making it all jagged. I think all my hole saws are for wood though. I'll do my best.