If you're trying to run poly fill and you're already running air to your tanks...a box filter is a stupid cheap option....unfortunately to get one that is of usable size you kinda have to DIY
It messy cause I'm in the middle of maintenence. This is one made out of a coffee can and some pvc. If you drink decaff you'll have a green filter and you can cut the upright tube level with the top of the can for a better/hide-able look.
Added small hole to lid using a soldering iron and used an exacto knife to trace/ cut a hole for upright tube, much easier to cut a 1 inch square and makes absolutely no difference on how well it works
Used a 1 inch to ½ inch reducer coupling as a bell end so water can get around the air stone just drilled some holes in the side. Then a ½ inch pvc upright tube.
Center the tube in the can and cover the bell with some lava rock. I like this stuff: https://www.homedepot.com/p/Mosser-Lee-1-5-Qt-Red-Lava-Rock-Soil-Cover-1140/204372347
Or any bio media
Top with poly and you're good to go.
Cheap and easy to make especially if you want to get a bunch of them going in multiple tanks