Yes, there are safe materials that you can use to make DIY canister filters and filter bags. You may not think an individual filter media bag is particularly pricey, but depending on the size, a filter media bag can cost as much as $5 for a single bag. Even smaller bags cost nearly $3 a piece. Not a huge amount of money, but if you have more than one aquarium, and you have to replace the media bags fairly often, the cost can add up. Using ordinary household materials, you can make media bags and canister filters at half the cost of what you'd buy them for.
Pantyhose Media Bag
For the same $5 you would spend on a single filter bag, you can purchase a pair of pantyhose or tights and make at least a half dozen or more filter media bags. Simply cut the hosiery to the desired size, leaving a bit of extra length to close the ends. Tie one end in a knot, or use a small rubber band to close the opening tightly. Fill the hose with the desired media, then close the other end with a knot or rubber band. Voila, you have your do it yourself media bag at a fraction of the cost of a store-bought bag. Depending on the size of your filter media, you can also use a fine nylon mesh material, such as tulle. Tulle is sold by the yard at fabric stores and is commonly used for making veils and dresses.
Cheesecloth Media Bag
Another do it yourself media bag option is fine cheesecloth. Because the cheesecloth has larger pores than hosiery or commercial filter materials, you'll have to use several layers, but ultimately it works pretty well. Fold the cheesecloth about four layers thick, add the media in the middle, then pull the ends together and tie them off by knotting them or securing them with rubber bands. Because cheesecloth is made with cotton, the material will eventually break down in water, so be sure to check it periodically and replace it if it begins to deteriorate. If the filter is left too long and it begins coming apart, pieces of the cheesecloth can end up in your pump.
DIY Filter Sponges
Another do-it-yourself option for filters is to use quilt cotton batting in place of sponges. A bag or a few yards of batting will provide you with numerous filter sponge pieces that can be cut to the desired size and shape, and at a fraction of the cost. Quilt batting is sold by the yard at fabric stores.
Cheap Filter Media
Lastly, instead of buying ceramic rings for your canister filter, consider using lava rocks. They are inexpensive and offer a huge surface area for bacterial colonization. Be sure to use only new lava rock, as a rock that has been used in landscaping may contain residual materials from lawn treatments that could add toxins to the water. You can buy lava rock by the bag at home centers and gardening centers.