DonoBBD, if you can find a spec curve for your pump, like the following example, and know the depth of the diffuser, then you will have a better idea of the cfm lost at depth. This is a graph of a linear diaphragm pump which looses volume very quickly at depth. Likely whatever you are using, not nearly as much.
Linear diaphragm pump The legend across the bottom is depth of water in feet.
