Aquarium filters are expensive, especially the replacement filters. Using things I had laying around the house, I made some of my own for under $.25.
Step 1: Commercial Filters
Commercial filters are expensive (unless they came with your tank at a garage sale, like mine).
Step 2: A Filter's a Filter...
I realized how simplistic those commercial replacement filters are, and found a substitute: air filters.
Now, these commercial filters come with carbon or other media as a chemical filter, so I just bought my own.
Step 3: Create a pocket...
Create a pocket for the chemical media. I simply hot-glued the edges.
Be sure to squish the glue between the filter parts.
Step 4: 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.
Step 5: There you have 'er.
This has been my first instructable. Give me criticism.