You will want to keep an eye on that. Maybe put them in a cooler with a bubbler during the test. Or, at least check on them every few hours to make sure they don't start piping at the surface due to low DO levels. If the non-waterfall pump does not make any splashing, but just moves water around...leaving that one on would not make much difference.
How big are the fish and how many gallons of water are in the pond?
Consider putting a rain barrel out to catch water from the gutter downspouts. You would not have to de-chlorinate that!
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