Bob,

Glad to hear that your water well is probably going to be "supplemental" for the pond. (That makes me feel a little better!)

20 GPM is way too much flow for a 3/4" garden hose, you will definitely be wasting lots of effort from your submersible pump. (Even a 3/4" tap is a restriction. If they are already going to add a second hydrant, perhaps consider paying a little more and having them match one tap of the hydrant to the same size as the incoming pipe? Extra flow rate available for pumping to a pond, or fighting a grass or house fire is pretty valuable IMO.)

I also agree with Imoore above. Once you already have moving water, you might as well aerate it! I recall some old threads on the forum discussing easy means of aerating in that situation.