Update: Since the Fusion 700 air pump is no longer available, the Danner AP-4 is now recommended for most scrubbers, except the SURF4 or 4x which can use a Danner AP-8, and the SURF8 which needs a large single-outlet pump.

Note that all HOG and DROP models have 1 air inlet, therefore the 2 outlets of the Danner AP-4 pump can be combined into 1 with a "T" or "Y" fitting, and this will give you a lot of air which can still be reduced with the control knob. If you don't want to combine the outlets, then the smaller Danner AP-3 pump has only 1 outlet, but it also does not have a control knob.