It seems that there are always questions in the fall about winterizing plastic ponds. To begin with, clean out all the gunk (composed of fish and plant waste) at the bottom of the pond. Specialist garden catalogues have a little gizmo that attaches to a hose and when the hose is run, the gizmo acts like a vacuum cleaner, sucking debris from the bottom of the pond. Or you can put your pump on the pond bottom and point the discharge into the garden.

If you don't remove the plant debris, it will continue to decompose. Decomposition uses oxygen as one of its primary fuels and this means that oxygen will be taken from the water to fuel plant decomposition. If there is an ice-layer over the pond, and there will be shortly, the water will not be able to replace that oxygen and the pond will go into an anaerobic (without oxygen) state under the ice.

Now, you've never quite smelled something until you've taken a whiff of a pond that's in that state. It is basically your very own backyard sewage system. Aside from getting rid of the smell, the reason you remove the bottom layers of material is so any fish you're leaving in the pond will have enough oxygen to survive the winter. And survive they will as long as you stop feeding them when the water temperature is less than 50F.

At that temperature, it is really too cold for them to feed and any food will simply rot. The fish will survive as long as the water doesn't freeze solidly to the bottom of the pond. If the pond is three to four feet deep, it will not freeze and your fish will be fine. Shallower ponds will either have to have a bubbler, a pump left running to keep an open area open or the fish removed to an aquarium for the winter. But start with removing the gunk.