Stinkyattic's got it right! I think the vent-hole question is in regards to a light-baffle, something to prevent light from leaking into the ventilation hole.

A cheap solution is to install a PVC elbow joint (painted black) the same diameter as the hole, oriented so as not to let light in.

Whether you install a carbon-filter is up to you and how much the plants stink.

Good luck :thumbsup: