The line actually goes into the bottom of the internal filter through a course sponge, through some filter wool then into the powerhead. By the time it comes out it's pretty much the same size as the micro bubbles that come out of a ceramic diffuser.
look at me getting all defensive :slfg:
Yeah, I'm getting the little bubbles everywhere at the moment. The tank looks a bit cloudy because of them. That and I haven't sorted out the overflow to suck the film off the surface (it's halfway built, just need the sump return plumbing) so the bubbles all sit under it until they get blown around a bit by the single outlet that's rippling the surface.