During the winter when the pond surface freezes, toxic gasses are formed by decomposing organic debris. The gas is trapped for prolonged periods of time because it cannot escape through the ice cap. These gasses are very damaging to the inhabitants.
Although it is important all year, Fall is a particularly good time to clean your pond thoroughly, removing any accumulated muck from the bottom. (It is in the Fall that more debris is dropped into the pond from trees shedding leaves and blowing storms.) Cleaning will leave as little decaying potential as possible.
However, no matter how clean, there will still be gas build-up under an ice-covered pond. The best and easiest solution for ice-over is to use a pond heater or de-icer. Although "pond heaters" are not made to actually heat your pond (not like a swimming pool or aquarium), they ARE made to keep a gas exchange hole open during times of freezing surface. Read More HERE