Reel McKoi wrote:

When they die here in the winter in shallow ponds, and a bubbler us used,
they claim it was hyper chilling. Disturbing the water layers until the
bottom layer was too cold for the fish to survive.

Hal's advice on bubblers is exactly what I would have given.

If there's no heater wouldn't the bottom warm layer mix with the colder
upper layer and "chill" the bottom to the same temperature as the top
(near freezing) layer?

Since I don't believe that you can get stratification in winter in a 15"
deep pond, it makes no difference (to temperature) whether a bubbler is
used or not. The bubbler exists just to keep a hole open, and . The
concept of "hyper chilling" makes no sense to me. Goldfish will survive
water right down to the freezing point (with the odd claim that they
actually survived freezing - but I think there'd be lab experiments to show
that if if really could happen). You can't make a pond get colder than
that without salting it (noticeably more than the prophylactic salting many
use), and a bubbler would actually make it even _less_ likely (try super
cooling water at home - you can do it, but stirring it will make it freeze
immediately).

15" is (barely) deep enough to keep from freezing in most parts of Michigan,
most winters.
--
derek