Why Rainwater Beats the Tap Every Time

If you keep a pond, you will spend a good part of the summer watching the water level drop. Warm days, thirsty plants and a bit of evaporation can strip a couple of centimetres off a small pond in a week, and more during a proper heatwave. The temptation is to reach for the hose, but tap water brings things your pond does not want.

UK tap water is treated with chlorine or chloramine, and it often carries a fair amount of dissolved nutrients and minerals. Chlorine is tough on the microscopic life that sits at the bottom of your pond's food web, and it can irritate amphibians and fish. The minerals push your pH upwards over time, which suits algae far better than it suits most native pond plants and creatures. Rainwater, by contrast, is soft, slightly acidic, and free of chlorine, so it slips into your pond without tipping the balance.

There is a second reason to favour rainwater: it is what your pond has evolved to expect. Every natural pond in the country is topped up by rain, so using it keeps your water chemistry closer to the wild conditions your wildlife already knows.

Setting Up a Water Butt That Earns Its Keep

A standard water butt linked to a downpipe will fill surprisingly quickly. A modest roof can send well over a hundred litres into the butt during a single decent downpour, so even a small one will see you through most of the summer if you use it sensibly.

• Site it near the pond but not so close that a leak or overflow floods the surrounding soil. Two or three metres is usually plenty.
• Fit a diverter into the downpipe so the butt fills automatically and overflows back into the drain once it is full.
• Raise the butt on a sturdy stand. Gravity is your friend, and every 10 cm of height gives you a little more pressure for a hose or watering can.
• Cover the top. A lid keeps out leaves, keeps the water dark, and stops mosquitoes and midges from breeding in the butt itself.
• Check what your roof is made of. Water from a mossy or bitumen-covered shed roof is best avoided for the pond. Most modern tiled or slate roofs are fine.

If you can, run a second butt in series. The first fills and overflows into the second, which roughly doubles your storage for very little extra cost.

How Much to Add, and How Often

Small and often is the rule. A pond that loses a centimetre a day does not need a dramatic rescue; it needs a gentle top-up. Aim to keep the level within a couple of centimetres of your normal mark rather than waiting until the water has dropped noticeably and then adding a large volume in one go.

A sudden dump of cold rainwater into a warm summer pond can cause a temperature shock. Amphibian tadpoles, newts and many invertebrates are sensitive to that, and fish will sulk at the bottom for days. It also stirs up sediment, which releases nutrients into the water column and gives blanketweed a head start.

For a small pond, a few litres at a time from a watering can is ideal. For a larger pond, a slow trickle from a hose fed by the butt works well, provided the butt is high enough to give you a flow. If it is not, a small submersible pump on a length of hose will do the job gently.

Adding Water Without Waking the Neighbours

The trick is to add water where it will do least harm. Aim for the deepest part of the pond rather than the shallows, and pour onto a flat stone, a piece of slate, or a saucer sitting just below the surface. This breaks the fall and spreads the flow, so you avoid a plume of silt and a shock wave through the water.

• Do it in the evening or on a cloudy day. Cooler air means less temperature difference between the butt and the pond.
• Top up from the same spot each time so you get used to how the pond responds.
• Watch the wildlife as you pour. If newts or tadpoles scatter, slow down. If they carry on as normal, you have got the flow about right.
• Never let the butt run dry mid-pour and suck up the last murky dregs at the bottom. That sludge is best tipped onto the flower beds.

What to Avoid

Rainwater is the ideal top-up, but a few habits undermine it. If you must use tap water in an emergency, let it stand in an open container for 24 hours first so the chlorine can dissipate, and add it slowly. Do not use water from a hot tap, a water softener, or a condenser tumble dryer.

Watch out for runoff too. Water that has run across a driveway, a treated patio or a freshly fertilised lawn can carry oil, salt or nutrients straight into your pond. If your butt is fed from a roof where you have recently used moss killer or a roof treatment, divert the first flush elsewhere for a few weeks.

A Few Seasonal Pointers

Spring and early summer are when demand rises fastest, as plants take off and evaporation increases. Keep an eye on the level after any warm, breezy spell. Late summer is often the pinch point, so top up generously after any heavy rain to bank water for the dry weeks ahead.

Autumn and winter are mostly about overflow management rather than topping up. Make sure your butt can overflow safely and that the pond has somewhere to go if it fills to the brim, so the edges do not get scoured out. Come February, when frogs and toads are heading for the water, a full pond is exactly what you want.