Have you tried aerating a bucket of the water and then retesting?
water directly from the well has ZERO oxygen, if you just put them in the well water they will suffocate.
when you add it to the pond it gets diffused and get "oxygenated".
I have an arteasan well I use in My pond, I have it angled up so it splashes and Bubbles on the surfice as it enters the pond to increase the aeration of it. The more splashing and bubbleing the better.
