Oxygenating Pond

Fish breathe oxygen dissolved in the water, and in warm weather how much is dissolved limits how much stock a pond can carry. An oxygenating pond is a small shallow bay beside the main pond, held apart by a screen or narrow neck, through which water passes on its way in. It is planted thickly, and in water this shallow the rooted plants and the algae make most of the oxygen, rather than the free-floating plankton that does so in deeper water. The same planting grows plankton and small creatures that reach the main pond as fish food.

The gain comes from those plants, not from the open air. A small sheltered bay takes up less oxygen from the air per square metre than a wide wind-blown pond, roughly 1.5 against 4.8 grams a day, so sheltering it costs more than it returns. A harder limit sits over the practice: water holds less oxygen the warmer it is, about 14.65 milligrams per litre at 0 C (32 F) against 7.44 at 30 C (86 F), and shallow water warms fastest. The bay pushes water closer to whatever that temperature allows; it cannot lift the ceiling. Because it all depends on the bay staying shallow, a settling basin upstream is not optional: silt here is not a nuisance but the way it fails.

Plants that release oxygen by day consume it all night, so a heavy stand in a small volume can deliver water poorer in oxygen than the pond it feeds by sunrise, the hour fish are already most at risk. Output swings from oversaturated in the afternoon, itself hard on fish, to depleted before dawn. A sudden die-off is worse again, which is why no weedkiller goes in, and a floating mat over the top shuts off exchange with the air. Ordinary pond advice is to remove shallow planted margins for these reasons, so this works only as a bounded feature with the pond kept deep around it. Keeping it means lifting silt out yearly, thinning before the surface closes, and keeping the neck clear.