Having your own aquarium requires a lot of maintenance. One of the most expensive components of this continued maintenance is, of course, the disposable filter cartridges. Depending on the type of aquarium filter you use, you may be able to make your own reusable fish filter cartridges.
In the photo above, the filter on the left is a store bought disposable. The filter on the right is a reusable cloth filter I made. If your filter uses a similar filtration cartridge, you can make your own reusable one!
With a disposable filter cartridge as your template, use some readily available (non-fusible) quilt batting making sure the batting is a polyester or other non-natural material, which most batting is these days. Then, with a sewing machine or serger, simply sew a 1/4 inch seam around 3 of the sides- again, making sure to use a polyester or nylon thread (natural fibers will eventually break down in an aquarium environment).
Make a couple of these to rotate through, and simply wash after use. You may still have to purchase some activated carbon for use in your reusable filter, but carbon is generally available in bulk and for much less cost than the individual carbon packages that come with filter cartridges.
Just one less item to buy. And one less item to throw out.