Topping up a pond with rainwater

Rainwater is the RHS default for filling and topping up a pond. Tap water is the backup, used slowly, because of what is dissolved in it.

What the RHS says

On the pond care page and the construction guide:

• Mains water contains nutrients that encourage algae. Fill a new pond with rainwater from a water butt when you can.
• If you have to use the hose, warm the water in the sun first and add it in small amounts, so you do not shock the pond with a sudden cold top-up.
• A wildlife pond with no fish usually does not need topping up. Most wildlife copes with the level going up and down. A liner showing at the edge can be hidden with stone or plants, which also keeps sunlight off the liner.
• Seasonal ponds that hold water in winter and shrink in summer are a normal British pattern. The wildlife guide says this can suit newts, which can survive in the mud, unlike the fish that would eat their young. Do not rush to keep a wildlife pond brim-full in a drought.
• A fish pond is different. Fish need the water to stay deep enough and oxygenated. A sudden drop in winter can mean a leak. The RHS says to keep a leaking pond topped up until you can repair the liner in spring.

Chlorine is the other reason people avoid a straight hose. The RHS frames the problem as nutrients and temperature more than as a chlorine lecture. Either way, a water butt is the better barrel to dip.

How much rain you are likely to catch

A butt only helps if the roof fills it. Rowanspring's guide to garden rainwater storage works through roof yield, which is the practical limit on topping a pond up from the house. A tiny shed roof will not refill a large fish pond in August. Knowing that in April is more useful than discovering it when the liner is showing.

Keep the butt covered so it is not itself a drowning risk for a child or a hedgehog, and so it does not fill with leaves. The pond safety rules apply to any deep container, not only the pond. See pond safety for children.

What not to tip in

Do not empty a hot tub, a washing machine or a water-change from an aquarium into the pond. Soaps, salts and medicines are not rain. Do not scoop water from a neighbour's pond to "seed" yours. The RHS says that can bring disease and invasive plants.

If the pond is losing water faster than evaporation explains, look for a puncture or a wicking edge. The RHS wildlife page warns that turf laid over the liner will draw water out quickly in hot weather. Stop the grass short of the edge.

Is tap water safe for fish in an emergency?

The RHS allows small, gradual additions when rainwater is not available. If fish are already distressed, topping up is not a diagnosis. Ask a vet or an aquatic specialist.

Can I use a water butt that has had a wood preserver rinsed into it?

No. Only rain from a roof you would be happy to put on plants. If the butt has held chemicals, do not use it on the pond.