Good afternoon. The water polishing is done using a special micron cartridge. It filters out much smaller particles that any other filter can trap (including canister, hang on backs, wet drys). Too with this type of filter you can add what is called diatom powder, this will even filter out some
ammonia to a point, parasites and all kinds of cooties

lol I highly recommend them. Diatom filter is another great one for the same thing. Diatom powder is actually called diatomaceous earth. It's especially great when you have multiple tanks.