This week I went to look at Mount Pond in Clapham, after residents raised concerns about its condition. There has been talk of the water level being too low, which it may be, but standing at the edge in the July heat the more obvious problem was that the water looked starved of oxygen. A green film covered much of the surface, the water was still, and it had the look of a pond heading for trouble in a hot, dry spell. It is a good, visible example of something that quietly kills fish across the country every summer, so I wanted to explain what is happening, what to do in an emergency, and how it can be prevented.
Mount Pond, Clapham, July 2026. A green covering across a still surface in prolonged hot weather.
Fish can die in hot weather because warm water holds less dissolved oxygen, while the fish and everything else in the pond need more of it.
In a small or sheltered pond with no wind or water movement, oxygen can fall far enough to suffocate fish, and the warning sign is fish gathering at the surface and gasping, especially at dawn.
If you see this, call the Environment Agency incident hotline on 0800 80 70 60 straight away. The lasting prevention is to keep the water circulating and aerated, usually with a pump.
Why fish die in hot weather
Water holds oxygen in dissolved form, and fish breathe that dissolved oxygen through their gills. The crucial fact is that warm water holds less dissolved oxygen than cold water. So as a pond heats up through a hot spell, the amount of oxygen available falls at exactly the time the fish, and the bacteria and plants breaking down waste, are all respiring faster and demanding more of it. The supply drops as the demand rises.
It gets worse overnight. During the day, submerged plants and algae release oxygen through photosynthesis, but after dark they stop producing it and instead consume it, so dissolved oxygen is usually at its lowest just before dawn. That is why fish in a struggling pond are often seen gasping at the surface early in the morning, trying to reach the thin oxygen-richer film at the very top. Fish crowding the surface, gulping air, is the clearest warning that oxygen is running out, and it is a sign to act rather than wait.
What to do in an emergency
If you see dead fish, or fish gasping at the surface, call the Environment Agency's 24/7 incident hotline on 0800 80 70 60 immediately. The Agency's fisheries officers investigate these reports and can deploy aeration equipment, divert fresh water, or rescue trapped fish before more die. Reporting it early gives them the best chance of saving the fish.
In a genuine fish-kill situation the Environment Agency uses specialist oxygenation kits: high-output pumps and aerators that drive oxygen into the water quickly to lift dissolved oxygen back to a survivable level. It is an emergency intervention, and the fact that it exists tells you how serious low oxygen is. The better position, of course, is never to reach that point.
What aeration actually is
Aeration simply means introducing oxygen into the water, and in nature it happens by physical means. Wind blowing across a large body of water creates ripples and waves that break the surface and fold air into the top layer. Rainfall does the same as it strikes the surface. Anything that disturbs and breaks the surface increases the transfer of oxygen from the air into the water.
This is why large lakes usually look after themselves. They have enough open space, or fetch, for wind and currents to keep rippling the surface, so oxygen keeps being drawn in. A small, sheltered or tree-lined pond does not get that. In still, hot weather the surface goes flat and glassy, the water stratifies into a warmer top and a cooler, oxygen-poor bottom, and dissolved oxygen in the pond as a whole falls away. Mount Pond, sheltered and still in the heat, is exactly this situation, and it is the situation a pond owner needs to design against.
Algal blooms, and the duckweed feedback loop
The same hot, still, nutrient-rich conditions that lower oxygen also trigger surface growth, and that growth then makes the oxygen problem worse. It is worth being careful about identification here.
A note on identification: the green covering in these photographs appears to be a floating growth, potentially duckweed or watermeal, possibly mixed with fine surface algae. This cannot be confirmed from photographs alone, and some potentially harmful algal blooms can look similar. Caution should therefore be exercised, and specialist identification or water testing should be sought wherever water quality, or human or animal safety, is a concern.
One common culprit is duckweed, and once it takes hold it feeds the very problem that created it. A dense surface covering of duckweed forms when prolonged hot, sunny and still weather creates ideal conditions for explosive growth. These tiny free-floating plants can double their population in a matter of days, thriving in nutrient-rich water where there is no wind or movement to break them apart. Though the plant itself is natural, a thick mat poses several real risks to the pond:
The trap is the feedback loop: low oxygen and excess nutrients encourage the growth, the growth then blocks light and oxygen and adds to the depletion, which stresses the pond further. The RHS has useful guidance on duckweed and its management. It is also why the nutrient side matters at the catchment scale: excess phosphorus and nitrogen are what feed these blooms, which is exactly why considerations such as nutrient neutrality are applied to new development, so that building does not add to the load a water body already struggles with.
A changing climate makes this more common
Hotter, drier and more prolonged summer spells make low-oxygen events more frequent, and the wider pattern of climate variability, including cycles such as El Niño, is part of that picture. We looked at how these shifts affect water and pumps in our article on El Niño, extreme weather and water. The practical takeaway for anyone with a pond is that the still, hot conditions that cause fish kills are likely to occur more often, not less, so it is worth designing for them rather than reacting each summer.
The prevention: keep the water moving
If the problem is that a still surface stops exchanging oxygen, the solution is to keep the water moving so the surface is constantly broken and refreshed. In practice that means a pump. A pump that circulates the water, and better still one that lifts water and lets it fall back through the air as a fountain or cascade, does artificially what wind does naturally on a large lake: it breaks the surface and folds oxygen in. It also stops the stratification that traps a cold, dead layer at the bottom.
Our guide to how to aerate a pond with a pump explains the methods in detail. The other question people ask is whether the pump should run constantly, and in hot weather the answer is generally yes, which we cover in should a pond pump run all the time. The honest reality is that almost all ponds and lakes benefit from keeping the water circulating year round, because it avoids the panicked, expensive interventions that a summer oxygen crash forces on you.
How much does a pond pump cost to run?
Because a pond pump earns its keep by running continuously, running cost is a fair question, and the answer is usually reassuring. Take a small pond pump such as the APP TPS at 200 watts. At the time of writing the average UK electricity price is around 23.5p per kWh. A kilowatt-hour, or kWh, is the standard unit of energy: it is 1,000 watts running for one hour. A 200 watt pump therefore uses one kWh every five hours, so five hours of running costs about 23.5p. Over a full 24 hours it uses roughly 4.8 kWh, which is about £1.13 a day, or around £34 a month, to keep the water oxygenated and clear. Set against the cost of losing a pond full of fish, or of an emergency call-out, continuous circulation is cheap insurance.
Matching circulation to your water
The right pump depends on the size of the water body and how much surface movement you need. This is a quick orientation rather than a full buying guide; for choosing on volume, turnover, solids and head, see our guide to what is the best pond pump and how to choose one.
| Water body | For circulation and aeration, start with |
|---|---|
| Small garden or fish pond | APP TPS or TPV, continuous-duty circulation for small ponds |
| Medium pond, feature or cascade | APP MH, more flow for stronger movement and waterfalls |
| Lake, large pond or big waterfall | APP HD-15, high flow for large-scale circulation and cascades |
| Changing water level, or an instant floating fountain | APP Floating Tree, floats and aerates the surface, see our large lake fountain guide |
Back to the pond in the photographs
Mount Pond, Clapham, July 2026. Still, flat water with a green surface growth, and no wind movement to break it up.
So what would actually help the pond pictured? It would clearly benefit from circulation and the aeration that comes with it. A few things shape the answer. The water level changes, so a fixed pump plumbed to a set pipe height, such as a submerged MH feeding a fixed fountain, would be awkward and could end up exposed or submerged as the level moves. A very small pump, meanwhile, would not disturb enough of the surface to make a real difference on a water body this size. That leaves two realistic routes: a substantial surface-mounted pump circulating water around the pond, or a floating fountain.
My recommendation here would be a floating tree. It floats on the surface and rises and falls with the water level, so the changing depth is no longer a problem. It would give some immediate relief, circulate the water, and as the fountain spray lands back on the surface it breaks it up, driving oxygen in through aeration and moving the pond back towards health.
Floating Tree Pond pump in use with high fountain over pond
Why I wrote this
I have written this in the hope of helping others manage their ponds through all weathers. We are often known for flood management, but my interest is in all things water, environmental and engineering, and understanding the process at work here is the key to managing it and keeping the wildlife healthy.
Pond and lake health depends on many factors, from planting to nutrient load to depth, but aeration and circulation sit at the heart of it, and a water pump is the most reliable way to provide them. If you would like a pump for your pond or lake, browse our pond and water feature pumps, or call the team on 0115 987 0358 to order.
Frequently asked questions
Why do fish die in hot weather?
Because warm water holds less dissolved oxygen than cold water, while the fish and the pond's bacteria and plants all demand more oxygen as temperatures rise. In a still, sheltered pond the oxygen can fall far enough to suffocate the fish. Levels are lowest just before dawn, which is when fish are most often seen gasping at the surface.
What should I do if I see fish gasping or dead fish in a pond?
Call the Environment Agency's 24/7 incident hotline on 0800 80 70 60 immediately. Their fisheries officers investigate fish-kill reports and can deploy aeration equipment, divert fresh water, or rescue trapped fish before more die. Reporting it early gives the best chance of saving the fish.
Does my pond need a pump to keep fish alive in summer?
In hot weather a pump that circulates and aerates the water is the most reliable way to keep dissolved oxygen at a safe level, especially in a small or sheltered pond where wind cannot break the surface. Large open lakes may aerate naturally through wind, but most garden ponds benefit from continuous circulation to avoid a summer oxygen crash.
How much does it cost to run a pond pump continuously?
A small 200 watt pond pump uses about one kWh every five hours. At an electricity price of around 23.5p per kWh, that is roughly 23.5p for five hours, or about £1.13 a day to run around the clock, which is approximately £34 a month. Larger pumps cost more to run, but continuous circulation is inexpensive relative to the cost of losing fish.
Why does duckweed or algae cover a pond in hot weather?
Prolonged hot, sunny and still weather, combined with nutrient-rich water, creates ideal conditions for rapid growth, and duckweed can double in a matter of days. A dense mat then blocks sunlight and seals the surface, cutting oxygen further and feeding a loop that stresses the pond. Excess nutrients from waste or run-off are usually the underlying cause.
Will a fountain help oxygenate a pond?
Yes. A fountain lifts water and lets it fall back through the air, breaking the surface as it lands, which increases the transfer of oxygen into the water. A floating fountain is particularly useful on a pond or lake with a changing water level, because it rises and falls with the surface rather than needing fixed pipework.
