Sanitary air filter that filters down to .023 micron at 99.99% efficiency. Use with 3/8" tubing (1/4" will also work). Also fits in a drilled rubber stopper for an easy waterless airlock.
Don't get it wet though (or feed it after midnight), it will never be the same. Store in a clean dry airtight place (zip top bag with a paper towel in it seems to work well for us) with
a small piece of aluminium foil over the holes. There is no real in or out side to it, but we recommend drawing a little arrow or two on the side to show airflow and always use it
in that direction. Should last a year or more if you take care of it.