The Plant Lady. Nelson Water Gardens and Nursery

By them I mean Anacharis, Cabomba, and Hornwort which go by the name "Oxygenators" in the aquarium trade. We call these plants "Submerged Plants" because the term "Oxygenators" is misleading for pond owners. While it is true that these plants produce oxygen during the day (if you look closely at these plants on a sunny day you can see the tiny bubbles of oxygen forming on the leaves) they also consume oxygen at night. So there is no significant gain in oxygen levels in a pond over a 24 hour period.

During the night both plants and fish are consuming oxygen and by morning the dissolved oxygen level in the water will have dropped to its lowest level. This fact is important to know because warm water "holds" less dissolved oxygen than cold water. Early morning during the summer is the critical time to determine if you are approaching the maximum "fish load" in your pond. If your fish are gasping at the surface in the early morning, it means the oxygen levels have dropped to the point that the fish are nearly suffocating. Once sunlight hits the pond the plants start producing oxygen by photosynthesis and the problem disappears...until the next morning.

If you observe fish gasping during the early morning hours, then you need to take steps to save your fish. You can add oxygen to the pond by adding an air pump, an air rock, or more "splash" via a new fountain, waterfall, or simply allowing the pump to bubble at the surface.