Step 4: Fill with chemical filter media

Picture of fill with chemical filter media
Now, prior to just making my own filters, I did use commercial filter replacements without the plastic frame that comes molded to them (I don't know why there was no plastic frame; I got them free). However, those puffed up with use.

I recommend putting a staple in the center of your new filter to keep it from puffing up like that.

Add chemical filter media.
Or you could reuse the plastic cage from the old filter after cleaning it- I used to do that regularly when I bought the ones without the plastic.