I don't remember which exact product it is but it is your typical highly porous mechanical sponge that many canister filters use. Any petland/petco would sell them as replacements for canister filters. I put it close to the return pump and it did a great job of dispersing the microbubbles that were getting into my DT. I rinse it out every now again but my filter sock catches most of the detritus before it reaches this point. I wouldn't use a fine filter pad as it may collect too much detritus.