If everything is running like it should be, an additional air pump shouldn't be necessary. If you want to run an airstone for decorative or other purposes, then you would need to get an air pump.
Ive had a mirabello 70 for a fortnight now, just started to use the built in air feature yesterday.....
theres a piece of tubing and a restrictor supplied, fit this to the top of the pump ( little rubber cap in there with air written on it ) open the cap, nice little hole in there for air tubing..... adjust restrictor for preferred bubbles, et voila..........