Wooden air stone diffuser driven. Does the job, quiet, low power usage. Only drawback I can see is it needs to be fairly tall & that doesn't work for folks who like to put them in the sump under their cabinet. I don't have a sump, built a little stand to have it next to the display instead of hanging off the tank. I guess that's another drawback, clutter by the display. Oh well, it works for me.
