For the next step, weíll need a sealable plastic container, such as Tupperware. The piece of foam will need to fit without being scrunched, but should be as close a fit as possible. Place the sponge in the container and then fill it with water. Before sealing the lid on tight, we need to be sure to get all the air out of the foam. Once we do this weíre ready to seal that lid and put the container in the freezer.
After the foam freezes, we can remove it from the freezer. Immediately drill a hole into the top of the foam with a power drill using a drill bit slightly smaller than the tubing we plan to use. Donít drill all the way through the sponge, just about half way. Itís also important that we only pull one piece of foam out of the freezer at a time, otherwise they will defrost on us before we can drill them, which will cause the drill to catch and tear the foam instead of cutting it.
My personal favorite adaptation of this DIY sponge filter is to use undergravel lift tubes (see picture above). These work great for nursery or fry tanks. All you need is the foam, this article, the lift tubes, and an air pump. The combined cost of these materials is far cheaper than a power filter and just as effective. The foam could also be used to create a sponge filter that would connect to a submersible pump. Attach tubing (perforated by your drill) to the submersible pump and then insert it into the sponge to create a powerful sponge filter, similar to those used in all of my tanks for my undergravel jets. □