Pond lacking oxygen even though the pump is running: the hidden causes

This is a scene we often hear about at the Nodebais counter: “I don’t understand, my pump works perfectly, the filter runs day and night, and yet my fish come gulping at the surface early in the morning.” The pump is not at fault; it is doing exactly what it was designed to do. The problem is that a water pump moves water; it does not produce oxygen. Between what we believe and what is really happening beneath the surface, there are often several cubic metres of water that never see a single bubble.

A water pump is not an oxygenator

Dissolved oxygen comes from two sources: exchange with the air at the surface and photosynthesis by plants and algae during the day. A submersible pump that draws water from the bottom of a filtration well and sends it back through a pipe submerged 30 cm below the surface creates almost no gas exchange. The water circulates and is filtered, but its oxygenation remains the same, and is sometimes even slightly worse after passing through the filter.

What really introduces oxygen is a disturbed, renewed surface. A waterfall, a filter return that ripples the water, a fountain, or air-pump bubbles rising from the bottom bring oxygen-poor water back up into the open air. In the field, we regularly see ponds where the filter return has been submerged “for the noise”. The quiet gained comes at the cost of oxygen.

Dead zones: where water never passes

Even when properly adjusted, a pump creates a preferred circuit. Water takes the shortest route between the intake and the return, leaving the bottom, corners behind rocks, and cluttered planting shelves untouched. In a 1.50 m deep pond supplied by a pump placed halfway up, the lower layer can remain almost motionless for weeks.

In summer, this stagnation is worsened by stratification. Surface water warms, becomes lighter, and floats on cooler, denser bottom water. The two layers mix poorly. The top remains adequately oxygenated, while the bottom becomes depleted, and that is precisely where the sludge, waste, and often the fish seeking cooler water are found. A simple thermometer lowered on the end of a string is enough to confirm this: a temperature difference of a few degrees between the surface and the bottom indicates a stratified pond.

What silently consumes oxygen

Sludge is the most underestimated consumer. The bacteria that decompose leaves, excrement, and leftover food breathe day and night. Boyd and Tucker, in their reference work on pond water quality, point out that the oxygen demand of sediments can account for a major share of the total consumption of a body of water rich in organic matter. In other words, it is not always the fish that are short of air: they share what little is available with kilos of sludge.

Warm water contains less oxygen: water saturated at 25 °C holds around 8 mg/L, compared with approximately 11 mg/L at 10 °C. At the same time, the metabolism of fish and bacteria accelerates. The supply decreases while demand rises, which explains why so many "uneventful" ponds deteriorate during a week of heatwave.

The filter: ally or competitor?

A biological filter is a factory for nitrifying bacteria, and these bacteria need oxygen to transform ammonia into nitrite and then nitrate. As long as the filter is clean and well supplied, it is an excellent investment. When it becomes clogged, the sludge trapped in the foam or between the lava rocks begins to ferment. The water coming out is then lower in oxygen than the water entering, and it may carry a characteristic swampy odor.

Filters filled with lava rock are particularly affected: their porous surface is excellent for bacteria, but it also traps enormous amounts of fine particles. Rinsing it with pond water, never chlorinated tap water, and checking the flow rate at least once a month in summer prevent it from becoming an additional consumer.

Diagnose it within 48 hours

The most revealing test costs nothing: observe the pond at sunrise, two mornings in a row. This is when oxygen is at its lowest, after an entire night of respiration without photosynthesis. Fish gasping for air at the surface or crowding in front of the water return at 7 a.m., then swimming normally at 3 p.m., indicate a nighttime deficit. Then measure the temperature at the surface and near the bottom, probe the sludge with a handle, look for rising bubbles, and check the filter's actual flow rate. In the vast majority of cases we see when advising people, two or three of these factors are responsible at the same time.

Correct: put air where the pump does not

First measure, free of charge: direct the filter return so that it clearly ripples the surface. Second measure, the most effective: install an air pump with one or more diffusers placed on the bottom, in areas the pump does not reach. The bubbles bring deep water up, break up stratification, and renew the surface. Let it run at night; that is when it is most useful.

Third measure, over time: reduce the organic load. Reduce feeding during hot weather, remove leaves before they sink, and vacuum or biologically break down the sludge in small stages rather than all at once, so you do not suddenly resuspend sludge that would consume even more oxygen.

You may also like

For further reading: Understanding and using active oxygen for a healthy pond.

READ THE ARTICLE

Frequently asked questions

— Is a waterfall enough to oxygenate a pond?

It helps greatly at the surface, but it does not circulate the bottom. In a deep or sludge-filled pond, an air diffuser placed on the bottom is still necessary.

— Should I leave the air pump running at night?

Yes, it is even the most important time: oxygen levels drop all night and reach their lowest at daybreak.

— Why do my fish run out of air only in summer?

Warm water holds less oxygen while fish, bacteria, and sludge consume more. Stratification also isolates the bottom.

— Will a larger filter solve the problem?

Not necessarily. A filter improves water quality but consumes oxygen itself. Without an agitated surface or bottom aeration, the deficit persists.

In summary

• A water pump circulates water, but oxygen enters only through an agitated surface.
• The bottom, corners, and stratified summer water often escape the pump circuit.
• Sludge, a clogged filter, nighttime algae, and warm water deepen the deficit.
• Water return rippling the surface, an air diffuser on the bottom, and less organic matter: the trio that solves the problem.

Last updated: September 29, 2026.