Diaphragm pumps (also called air pumps) pump air into the aquarium/pond. The air enters the water through air stones and thus increases the oxygen content. Partly through the small bubbles dissolving oxygen into the water on their way to the surface, but mainly through air bubbles moving the water surface as they rise and thus increasing the surface area. The larger surface area enables a greater uptake of oxygen from the air.
If you’d like to buy a JBL PROSILENT diaphragm pump or are looking for additional information: Air pumps
Find out more about the technical options for aerating an aquarium here: Air pumps