You aerate a pond by moving water so that it breaks the surface. A pump driving a fountain, spray or waterfall disturbs the surface, and that disturbance is where oxygen actually gets into the water. It sounds simple, but understanding why it matters, especially in summer, is what keeps fish alive and a pond healthy.
Why ponds need aeration
Everything living in a pond needs oxygen: the fish, and just as importantly the beneficial bacteria that break down waste and keep the water clear. That oxygen is dissolved in the water, and it enters mainly at the surface, where air and water meet. The more you disturb and move the surface, the faster oxygen transfers in.
The problem is that dissolved oxygen falls exactly when demand is highest. Warm water holds less oxygen than cold water, and in a summer heatwave the fish and bacteria are more active and using more of it. Oxygen levels are lowest overnight and around dawn, because plants stop producing oxygen in the dark and start consuming it. This is why fish are often seen gasping at the surface on a hot, still morning. As our climate throws more extreme weather at us, these oxygen crashes are becoming more common, something we cover in our piece on El Nino, water resources and pumps.
How a pump aerates a pond
A water pump aerates in two ways at once. First, it circulates the water, which prevents warm, oxygen-poor layers stagnating at the bottom and keeps the whole pond mixed. Second, when it drives a fountain, spray or waterfall, it throws water through the air and breaks the surface, and that is where the oxygen exchange happens. A tall, fine fountain spray or a tumbling waterfall adds far more oxygen than a still surface ever could.
Which pumps to use
For most garden ponds, the APP MH range is the go-to. It is a continuous-duty pond pump built to run day and night, moving from 230 L/min on the MH150 up to 380 L/min on the MH750, and it will drive a fountain or a waterfall to break the surface and oxygenate the water. It handles solids up to 6mm, so pond debris does not clog it.
For a large pond or a lake where you want maximum surface disturbance and instant impact, the APP Floating Tree floats on the water and can create a spectacular fountain, aerating a big area with no fixed plumbing.
APP Floating Tree in action
For a fuller explanation of driving a cascade, see our guide on how to create a waterfall.
For very heavily stocked ponds, a dedicated air pump and diffuser adds oxygen at depth as well. But for the majority of garden ponds and features, a water pump that keeps the surface moving is an effective and attractive way to aerate.
When to run it
Pond pumps are usually run continuously, however in hot weather it is important to run the pump continuously, and overnight in particular, because that is when oxygen is scarcest. Pond pumps are rated for this continuous running, which is exactly why they differ from ordinary submersible pumps.
