How do I look after a pond in a heatwave?
Hot weather strips oxygen out of pond water while simultaneously increasing what everything in it needs. Aeration, shade and restraint — and the one thing never to do to a struggling pond.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 6 min
- Safety
- caution
Short answer
Warm water holds less oxygen and fish need more of it, so aeration is the priority: run a pump or fountain around the clock, or play a hose onto the surface to break it up. Shade a third to a half of the surface, stop feeding fish, and top up little and often with rainwater rather than dumping in a large volume of cold tap water.
The winter guide to ponds explains that ice kills fish by trapping gases rather than by cold. Summer is the mirror image and it is more urgent: the water holds progressively less oxygen as it warms, decomposition and respiration consume more of it, and the crisis often arrives at dawn, after a whole night in which plants have been respiring rather than photosynthesising.
Safety
Step by step
- Understand the oxygen problem.The amount of oxygen water can hold falls as temperature rises — warm water simply cannot carry as much. At the same time, fish metabolism, bacterial decomposition and plant respiration all speed up in warmth, so demand rises as supply falls. Oxygen is lowest just before dawn, because plants have been consuming it all night, which is why fish are usually found gasping first thing.
- Aerate, and keep aerating.Anything that breaks the surface and circulates the water adds oxygen: a fountain, a waterfall, an air pump with an airstone, or a filter outlet directed to disturb the surface. Run it continuously through the hot spell, including overnight, because the night is when it is most needed. This is the single most important action.
- Treat fish gasping at the surface as an emergency.Fish at the surface with their mouths at the air are not hungry, they are suffocating. Get aeration going immediately. If you have no pump, play a hose onto the water surface from a height so it splashes and entrains air, or repeatedly bail water up and pour it back from a height. Do not feed them.
- Shade part of the surface.Aim for roughly a third to a half of the surface covered by floating and marginal plants — water lilies are ideal, since the pads shade the water and keep it cooler while giving fish cover. Temporary shading with netting over a small pond also works. A fully shaded pond is not the goal; oxygenating plants need light.
- Stop feeding fish in very warm weather.Uneaten food decomposes and consumes oxygen, and digestion itself raises a fish's oxygen demand. In a heatwave, feed sparingly or not at all for a few days. They will not starve.
- Top up little and often, with the right water.Evaporation can drop a pond level noticeably in a hot week. Top up gradually rather than in one large volume. Rainwater from a butt is ideal. If using mains water, add it slowly, in small amounts, and be aware that chlorine and the temperature difference are both harmful to fish in quantity — a dechlorinator is worth having if you keep fish.
- Remove decaying material.Fallen leaves, dead blanketweed, spent flowers and any dead fish decompose and consume oxygen exactly when it is scarcest. Net out debris and remove blanketweed by twisting it onto a stick, leaving it beside the pond overnight so creatures can crawl back in.
- Deal with green water and blanketweed patiently.Heat, light and nutrients cause algal blooms, and a bloom is a symptom rather than the disease. More plants competing for the nutrients, more surface shade and less feeding all reduce it over weeks. Barley straw works slowly. Resist chemical treatments in a pond with wildlife — killing a large mass of algae at once consumes a great deal of oxygen as it decomposes, which is precisely the danger you are trying to avoid.
- Keep the wildlife route open.A pond in a heatwave is a vital water source for birds, hedgehogs and insects. Make sure there is a shallow beach, a sloping side or a ramp so anything that gets in can get out — and keep it usable as the level drops, which is exactly when a steep-sided pond becomes a trap.
Tips
- A cheap air pump with an airstone costs very little and is the best insurance a stocked pond can have. Leave it running for the whole hot spell.
- Deeper ponds cope far better than shallow ones, because the depth stays cooler and the volume buffers change. Anything under about half a metre deep will swing dramatically in a heatwave.
- Do not carry out a full pond clean-out in summer. It is an autumn job — doing it in heat removes beneficial bacteria and stresses everything at the worst moment.
- A container pond or a half barrel heats up extremely fast and needs shading. Move it into shade if you can lift it.
- Topping up with tap water repeatedly adds nutrients and hardness over time, which feeds algae. Rainwater from a butt avoids that as well as the chlorine question.
Common mistakes
- Feeding fish more because they are at the surface — They are at the surface because they cannot get enough oxygen, not because they are hungry. Feeding adds waste that consumes yet more oxygen.
- Refilling a pond quickly with cold tap water — The temperature shock and the chlorine both harm fish and invertebrates. Top up slowly, in small amounts, and use rainwater where possible.
- Using an algaecide during a heatwave — A large mass of algae dying at once decomposes and strips oxygen from water that is already short of it. It can kill the fish the treatment was meant to help.
- Turning the pump off at night to save electricity — Night is when oxygen is lowest, because plants respire rather than photosynthesise. If the pump runs at only one time, it should be overnight.
If it doesn't work
Fish gulping at the surface, especially at dawn
Cause: Dissolved oxygen has fallen below what they need — Fix: Aerate immediately — pump, fountain, or a hose splashing onto the surface. Stop feeding. Remove decaying material. This is urgent.
Water gone bright green within days
Cause: An algal bloom driven by heat, light and nutrients — Fix: Increase surface shade and plant cover, stop feeding fish, remove debris, and be patient. Do not use a chemical killer in a stocked or wildlife pond in hot weather.
Thick blanketweed everywhere
Cause: High nutrients plus strong light and warmth — Fix: Twist it out on a stick, leave it at the edge overnight for creatures to escape, and reduce nutrients over time with more planting and less feeding.
Water level dropping fast
Cause: Evaporation in heat and wind, not necessarily a leak — Fix: Top up gradually with rainwater. Before assuming a leak, remember that a hot week can drop a pond by a surprising amount through evaporation alone.
Smell of rotten eggs from the pond
Cause: Anaerobic decomposition in silt at the bottom, from oxygen depletion — Fix: Aerate strongly, remove accumulated debris and silt gradually rather than all at once, and improve circulation.
Questions people ask
Why are my fish at the surface gasping?
The water has run short of dissolved oxygen — warm water holds less, and everything in the pond is consuming more. It is most acute just before dawn. Aerate immediately by any means available, stop feeding, and remove decaying material. It is an emergency rather than something to watch.
Should I top up my pond in hot weather?
Yes, but gradually and little at a time. Rainwater is ideal. If you must use mains water, add it slowly in small volumes rather than filling with a hose, because of the chlorine and the temperature difference.
How much shade should a pond have?
Roughly a third to a half of the surface covered by floating plants such as water lilies is a good target. It keeps the water cooler, gives fish cover from predators and reduces algae, while still leaving enough light for oxygenating plants to work.
Is green water harmful to fish?
Green water itself is usually not directly harmful and often clears as planting establishes. The danger is a large bloom dying off suddenly, which strips oxygen as it decomposes. That is why treating it chemically in hot weather is risky.