Summer Pond Problems and How to Solve Them

Why Summer Puts Ponds Under Pressure

By July, a British pond is working hard. Water temperatures in a shallow garden pond can creep past 22°C during a warm spell, and every degree makes a difference. Warm water simply holds less dissolved oxygen, while at the same time fish, plants and the bacteria in your filter are all respiring faster and demanding more of it. Add long daylight hours, a steady supply of nutrients from fish waste and decaying leaves, and the odd thunderstorm that dumps rich run-off into the water, and you have the perfect conditions for green water, gasping fish and a pond that looks more like pea soup than a garden feature.

None of this is a sign that you have done anything badly wrong. It is simply the season doing what it does. The good news is that most summer pond problems have straightforward, low-tech fixes — and a few of them take nothing more than an hour on a Saturday morning.

Dealing With Green Water and Blanketweed

Green water is caused by microscopic algae floating in the water column. Blanketweed, that stringy filamentous stuff, is a close relative with a talent for clogging pumps and wrapping itself around marginal plants. Both are fed by two things: light and nutrients.

• Cut the light. Aim to cover roughly a third to a half of the pond surface with floating or lily-like foliage by midsummer. Water lilies, water hawthorn and floating heart all do the job beautifully, and they shade the water beneath without starving the pond of light entirely.
• Starve the algae. Scoop out fallen leaves, dead flowerheads and uneaten fish food before they break down. Every rotting leaf is a meal for the next algal bloom.
• Plant the margins. Irises, marsh marigold, sedges and rushes hoover up nutrients through their roots, competing directly with algae for the same food.
• Add oxygenators. Hornwort, elodea and water milfoil grow fast in summer, absorbing nutrients and sheltering tadpoles and small invertebrates as they go.
• Try barley straw. A small net bag of barley straw floated in the pond releases compounds as it breaks down that gently inhibit algae. It works slowly, so put it in early in the season rather than waiting for a crisis.

Be cautious with chemical treatments. Most algaecides knock the oxygen out of the water as the algae die, which is exactly the opposite of what your fish need during a hot spell.

Keeping Oxygen Levels Up

If your fish are hanging at the surface at dawn, gulping air, you have an oxygen problem. This is most likely to happen in the early hours, because aquatic plants produce oxygen only in daylight — overnight they consume it.

• Run fountains, waterfalls and air pumps through the night, not just the day. A small air stone placed near the bottom is far more valuable than a decorative spout that only stirs the top inch.
• Consider a solar-powered aerator for ponds without mains power nearby. Even a modest trickle of bubbles makes a real difference in a small pond.
• Never let a pump sit idle in still, hot weather. Moving water is oxygenated water.
• If fish are gasping now, do a partial water change of around 10–20 per cent in the cool of the evening using rainwater, and increase surface movement immediately.

Topping Up Without Making Things Worse

Evaporation is relentless in a heatwave, and a pond can lose an inch or more of depth in a week. Resist the urge to reach for the hosepipe. Tap water brings chlorine, chloramine and, in hard water areas, a hefty dose of the nutrients that feed algae. A sudden top-up can also shift pH and temperature quickly enough to stress fish.

Rainwater is always the better choice. Keep a water butt plumbed to a shed or greenhouse roof and use it for top-ups. If you must use tap water, let it stand in a bucket overnight to let the chlorine dissipate and bring it up to ambient temperature, then add it slowly rather than in one gush. Top up in the evening, never at midday, and avoid large water changes during a heatwave — little and often is far kinder to everything living in the pond.

Shade, Shelter and a Cooler Pond

If your pond sits in full sun all day, planting is your best long-term ally. A shrub or tall perennial on the south or west side will cast afternoon shade across part of the surface without dropping vast quantities of leaves into the water. A pergola or a strategically placed container of tall grasses does the same job in a smaller garden.

Depth matters too. A pond with a section at least 60–75cm deep gives fish a cool refuge when the surface warms up, so don't be tempted to fill a deep pond shallow for the sake of a few marginal plants.

A Few Summer Habits Worth Keeping

• Feed lightly. Fish eat less in very warm weather, and uneaten food is algae fertiliser. Feed in the cool of the morning and remove anything left after ten minutes.
• Deadhead and tidy. Remove spent lily flowers and yellowing leaves before they sink.
• Leave wildlife a way out. A gently sloping edge, a piece of stone or a ramp of rough timber lets hedgehogs, frogs and fledgling birds climb out if they fall in.
• Keep chemicals away. Slug pellets, lawn feed and weedkiller all find their way into ponds through run-off, and none of them do pond life any favours.

Do these small things and your pond will come through the hottest weeks in good heart — clear enough to see the tadpoles, busy with damselflies, and ready for whatever the rest of the year brings.