So I admit, I'm a bit of sponge addict! I LOVE sponges because I think they're extremely versatile and hugely utilitarian. Typically reserved for mechanical filtration in canisters, HOBs, and other mechanical filters, sponges are also widely used as bio-media for air driven filters. Every time I stop by my LFS, I make it a point to check out the sponge selection. I buy square sheet sponges, cylinder sponges, pond sponges, etc. I always have a need for the non-standard types. Because I'm a JIC (just in case) hobbyist, I will regularly buy them in the event a new use presents itself.
The photos above show some of the various sponges I keep on hand. These all vary in size, thickness, and density.
The cylinder sponges (upper left) are typically used to pre-filter intakes of canisters and HOBs. I even keep one on the intake in the corner overflow of my pre-drilled 75g.
The rectangular sponges (upper right) typically go in HOBs, but I sometimes find customized uses for them. The circular sponges (lower left) are for various canisters. The funky shaped black piece in that photo is what remains of a sheet sponge that I've cut up for various special projects. A full sheet sponge is shown in the lower right photo. This is actually pond sponge that comes in sheets 18" x 12" x 1". These are very handy when you need something very specific. You just cut it to fit what you need.
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub