Ok, 350' of hose would just be a heat exchanger and water would be the same or higher than pond temp by the time it got there.
Move diffuser shallower and this might be a good time to engineer an underground supply line to the second pond for drought AND temp control.
I'm not sure the 5gal ice chunk would make much difference, it would be like an ice cube from your ice maker in a 10gal container.
