NOT a dish washing sponge! Check your aquarium store, they likely have sponges for aquariums and I recommend staying with one made for filtration. A dish sponge has two negatives. The first and most dangerous is that they are often treated with soap or other chemicals. The second is that they are far too dense to allow the water to flow through as rapidly as you need to not overflow the tank. If your local fish store does not carry a good size (measure your opening before you shop) try looking on-line. Looking on eBay, an alternate name would be foam block and that term may be more helpful. You want it to have large "holes" verses the small ones of a household sponge, the coarser the better. Depending upon the opening a coarse filter mat might work but finding one thick enough to stay in place may be difficult. Scroll down to the picture of the black filter sponge in this eBay ad for a look at the texture of item 8133.