Does a pond have to run its pump 24 hours a day?

A filtered fish pond does, because the filter bacteria die without a constant flow of oxygenated water. A plant-only wildlife pond needs no pump at all, and a fountain used for looks can run on a timer.

How much does a garden pond cost to run?

Electricity is the main running cost. A modern 35 W pump running continuously uses roughly 300 kWh per year; an oversized 120 W pump uses about 1,050 kWh. Add a UV clarifier only for the algae season, and top up water losses in summer.

Why is my pond water green?

Green water is single-celled algae feeding on excess nutrients and light. The lasting fix is shade from floating plants over roughly a third of the surface, fewer fish, less feeding and enough filtration; a UV clarifier only treats the symptom.

Backyard Pond Guide — Plan, Build & Maintain a Garden Pond

Q: How deep does a backyard pond need to be?

A wildlife pond works from 40 cm. If you want fish to overwinter outdoors, plan at least 80 cm over part of the pond, and 120 cm in regions with hard frosts so a frost-free layer always remains at the bottom.

A pond is the cheapest way to double the wildlife in a garden

Nothing else you can add to a garden — no hedge, no feeder, no wildflower strip — brings back as much life per square metre as standing water. Frogs and newts arrive on their own, usually in the first or second spring. Dragonflies follow the frogs. Birds and hedgehogs drink from the shallow edge, and in a dry August that edge matters more than anything else you planted.

The catch is that a pond is also the one garden feature that fails loudly. Get the depth, the shade and the water volume wrong and you spend every summer fighting green soup, blanketweed and dead fish. Get them right at the planning stage and the pond mostly runs itself, with an hour of attention a month.

This guide is organised in the order the decisions actually have to be made: where and how big, then how it's built, then what machinery it needs — and only then what lives in it. Skipping ahead is how most expensive mistakes happen.

The four numbers that decide everything

Volume — under 2,000 L, water chemistry swings hard and fast.
Depth — 40 cm for wildlife, 80–120 cm if fish must overwinter.
Sun — 4 to 6 hours a day; more means algae, less means sad plants.
Turnover — a fish pond's pump should move the whole volume every 1–2 hours.

1. Planning: where, how big, how deep

Two afternoons spent watching your garden will save two summers of trouble. Track where the shade falls, where water already collects after rain, and where you'd naturally stand to look at the pond.

Choose the spot

Half a day of direct sun is the target. Keep the pond clear of deciduous trees — leaf fall in autumn is the single biggest source of sludge — and away from the drip line of anything that sheds needles or berries. Avoid the lowest corner of the garden if that's where runoff collects: lawn fertiliser and driveway wash are exactly what you don't want draining in.

Size it honestly

Bigger is genuinely easier. A large body of water heats slowly, cools slowly and dilutes mistakes; a small one boils in July and freezes solid in January. If space is tight, gain volume by going deeper rather than wider. As a rule of thumb, 4 m² of surface and 3,000 L is the point where a pond starts behaving like a stable little ecosystem.

Profile the depths

Design three levels: a shallow shelf of 10–20 cm for marginal plants and for animals to climb out, a mid shelf at 40–50 cm for water lilies in baskets, and a deep zone. At least one edge should be a gentle beach slope — a vertical wall drowns hedgehogs and traps frogs.

The three-level profile: shallow shelf, planting shelf, deep zone. The frost-free layer at the bottom is what keeps fish alive through winter.

Before you dig: call the utility-location service for your country and check for buried gas, water and electricity lines — a garden pond is typically 60–120 cm deep, which is exactly the depth at which service runs live. Check local rules too: many municipalities require fencing or a safety grid for standing water once children are on the property, and some limit surface area before a permit is needed.

2. Building it: liner, preformed shell or concrete

Three construction routes, and the choice is mostly about how irregular you want the shape and how long you want it to last.

Method	Best for	Lifespan	Watch out for
Flexible liner EPDM or reinforced PVC	Any shape, any size. The default choice for 9 ponds out of 10.	EPDM 30–50 yrs PVC 10–15 yrs	Never lay it on bare soil — one flint punctures it. Underlay or 5 cm of sand first.
Preformed shell rigid HDPE / fibreglass	Small ponds under 1,500 L, fast weekend installs.	15–25 yrs	Backfill in layers and compact as you go, or the shell shifts and the water level goes crooked.
Concrete poured or block, sealed	Formal geometric pools, raised ponds, walk-on edges.	50+ yrs	Cracks with ground movement; fresh concrete leaches lime and must be sealed and cured before any fish go in.

The build, step by step

Mark the outline with a hose or rope and live with it for a day.
Dig the deep zone first, then cut the shelves back into the walls.
Level the rim all the way round with a plank and a spirit level — a rim 2 cm out shows forever once it's full.
Remove every stone and root, then lay underlay (or old carpet, or 5 cm of sand).
Drape the liner loosely, no stretching, and let the weight of the water pull it into shape.
Fill slowly, easing out the big folds as the level rises.
Trim the liner leaving 30 cm spare, and hide the edge under stone, turf or gravel.
Wait before stocking — plants first, fish four to six weeks later.

Liner size, without the guesswork

Use the standard formula, adding 60 cm of overlap on each dimension:

Length = pond length + (2 × depth) + 0.6 m
Width = pond width + (2 × depth) + 0.6 m

A pond 3 m × 2 m and 1 m deep therefore needs a sheet of 5.6 m × 4.6 m. For volume, multiply average depth by surface area: the same pond holds roughly 3,600 L, and that number drives every pump, filter and treatment decision that follows.

3. Pumps, filters and the electrical side

A wildlife pond can run with no equipment at all. The moment you add fish, you're running a small permanent water-treatment plant in the garden — and that plant needs power, all year, outdoors.

The pump

Size it by turnover: the whole pond volume through the pump every one to two hours. A 3,600 L pond wants roughly 2,000–3,000 L/h. Buy on efficiency, not on maximum flow — a modern 35 W pump moving 3,000 L/h costs a third as much to run as an old 120 W unit doing the same job, and it runs 8,760 hours a year.

The filter

Two stages that do different things: mechanical media strain out solids and get rinsed, biological media host the bacteria that convert fish ammonia to nitrite and then to harmless nitrate. Those bacteria take four to six weeks to establish and die within hours if the flow stops — which is why a fish pond's pump runs continuously, winter included.

UV clarifier

A UV lamp clumps free-floating algae so the filter can catch them. It clears green water in about a week, but treats the symptom rather than the cause, and the lamp loses output after roughly 8,000 hours even while it still glows. Replace it every second season and switch it off outside the algae months.

Pond electrics are house electrics that happen to live outdoors

This is where most DIY pond projects quietly go wrong. A pump running year-round needs a permanent, properly rated outdoor circuit: an RCD (30 mA) upstream, IP-rated enclosures, cable buried at the correct depth and protected in conduit, and a switched, weatherproof point far enough from the water. An extension lead trailed across the lawn every spring is not a solution — it's the reason half of all garden ponds have a nuisance-tripping breaker by their third summer.

If you'd rather understand that side of the job than guess at it, the French home-technology magazine Sorel·Est is written by a working electrician-domotician and covers outdoor circuits, RCD protection, heat pumps and garden automation with actual wiring detail and actual prices — the kind of grounding that transfers directly to a pond pump, a filter cabinet or an automatic top-up valve.

Aeration matters more than people expect. Warm water holds less oxygen precisely when fish need it most, and a still pond at 4 a.m. in August is the moment losses happen. A modest air pump with one stone costs a few watts and removes that risk entirely. In winter, keep a small hole in the ice — float a ball or use a de-icer — so that trapped gases can escape. Never smash the ice: the shockwave kills fish.

4. Plants and fish, in that order

Plants are the filtration you don't pay to run. Stock them first, give them a season to root, and the pond will be clearer with fewer machines.

Oxygenators

Hornwort, water crowfoot, elodea. Submerged, they strip nutrients from the water column and starve algae of food. One bunch per square metre of surface.

Floating leaves

Water lilies, frogbit. Shade is the real algae control. Aim for coverage of about a third of the surface — more and the water below goes too cold and dark.

Marginals

Iris, marsh marigold, water mint. They live on the shallow shelf, hide the liner edge, and give dragonfly larvae a stem to climb when they emerge.

Keep out

Invasive species banned in much of Europe and North America — water hyacinth, parrot's feather, floating pennywort. Check your national list before buying online.

Stocking fish without overloading the pond

The old rule of one centimetre of fish per 25 litres is generous; halve it if you want an easy life. Goldfish, shubunkins and orfe suit a garden pond of a few thousand litres. Koi do not: they reach 60–80 cm, produce a startling amount of waste, and need 5,000 L each with serious filtration behind them.

Add fish only four to six weeks after the plants, a few at a time, floating the bag for twenty minutes to equalise temperature. Feed what they finish in five minutes and no more — uneaten food is the fastest route to a nutrient crash. Below 10 °C their metabolism stops: switch to a wheatgerm feed in autumn, then stop feeding entirely for winter.

Water numbers worth knowing

pH 6.5–8.5, and stable matters more than perfect
Ammonia (NH₃) 0 — anything above is an emergency
Nitrite (NO₂) 0 — the second stage of the cycle
Nitrate (NO₃) under 50 mg/L; plants consume it
KH above 4 °dH, the buffer that stops pH crashes
Oxygen above 6 mg/L, critical over 25 °C

5. What a pond really costs

Indicative figures for a 3,600 L lined pond with fish, filter and pump. Build costs vary with region and how much digging you do yourself; running costs are dominated by one thing — electricity.

Item	Typical range	Notes
Excavation (DIY vs. mini-digger)	0 – 400 €	Two days of hard work, or half a day hired. Factor in removing the spoil.
EPDM liner + underlay (5.6 × 4.6 m)	250 – 450 €	The one place not to economise.
Pump (energy-efficient, 3,000 L/h)	120 – 300 €	Efficiency here decides the next ten years of bills.
Pressure filter + UV clarifier	150 – 400 €	Only needed once there are fish.
Outdoor electrical supply, done properly	150 – 600 €	RCD protection, buried conduit, weatherproof enclosure.
Plants, gravel, edging stone	150 – 500 €	Plants are cheapest bought small in spring.
Running: pump electricity	35 W ≈ 300 kWh/yr	An old 120 W pump costs about 1,050 kWh/yr for the same flow.
Running: UV lamp (seasonal) + replacement tube	60 – 110 kWh/yr + ~30 €	Off in winter; tube replaced every second season.
Running: water top-up, test kits, filter media	40 – 90 €/yr	Evaporation in a hot summer runs to several centimetres a week.

Two thirds of the lifetime cost of a garden pond is the pump's electricity bill, which makes pond budgeting an energy question more than a landscaping one. Readers weighing that against the rest of the house — insulation, heat pumps, renovation grants, what a kilowatt-hour actually costs on their tariff — will find the same arithmetic worked through in euros in the travaux et maison & jardin sections of Sorel·Est, which prices projects with distributor quotes rather than round numbers.

6. The year, season by season

Roughly an hour a month, unevenly distributed: spring and autumn carry the work, summer is watching, winter is leaving it alone.

Spring

Restart and service the pump; clean the filter with pond water, never tap water
Replace the UV tube if it's on its second season
Divide and repot overgrown marginals
Start feeding once the water holds above 10 °C
Test ammonia and nitrite weekly while the filter re-cycles

Summer

Top up evaporation losses; let tap water stand or dechlorinate it
Pull blanketweed out by hand with a stick — twist, don't chop
Run extra aeration during heatwaves and at night
Deadhead lilies and thin the floating cover past a third
Feed sparingly; remove anything uneaten after five minutes

Autumn

Net the pond before the leaves fall — this one job prevents most sludge
Cut back dying marginals above the waterline
Switch to wheatgerm feed, then stop below 10 °C
Vacuum or scoop the bottom sludge while it's still warm enough to work

Winter

Keep a hole in the ice with a floating ball or de-icer — never break it
Stop feeding completely
Lift or raise the pump if the pond is shallow, to avoid chilling the deep layer
Bring vulnerable equipment and UV units indoors before the first hard frost

7. When something goes wrong

Symptom	Likely cause	What actually fixes it
Pea-green water	Single-celled algae feeding on excess nutrients and light	More shade and more plants, fewer fish, less feeding. UV as a short-term crutch.
Blanketweed strands	High nitrate and phosphate, strong sun	Twist it out weekly; barley straw extract; plant more oxygenators.
Fish gasping at the surface	Low dissolved oxygen, usually at dawn in hot weather	Aerate immediately, add a fountain or air stone, shade the surface.
Water level dropping	Evaporation, or a leak at the liner edge or a fitting	Stop the pump for 24 h. Still dropping? It's the liner. Stable? It's the plumbing.
Cloudy grey water	Stirred sediment or a bacterial bloom in a young pond	Wait two weeks; check filter media; stop over-cleaning it.
Smell of rotten eggs	Anaerobic sludge on the bottom	Remove the leaf layer, improve circulation, never let sludge build past 2 cm.

Frequently asked questions

How deep does a backyard pond need to be?

A wildlife pond works from 40 cm. If fish must overwinter outdoors, plan at least 80 cm over part of the pond, and 120 cm where frosts are hard, so a frost-free layer always remains at the bottom for the fish to sit in.

Does the pump have to run 24 hours a day?

In a filtered fish pond, yes — the filter bacteria die within hours without a constant flow of oxygenated water, and restarting the filter means re-cycling it for weeks. A plant-only wildlife pond needs no pump at all, and a purely decorative fountain can run on a timer.

Can I fill the pond straight from the tap?

For the initial fill, yes — then wait. Let it stand a week before adding plants and four to six weeks before fish, so chlorine gasses off and the biological filter establishes. For top-ups larger than about 10% of the volume, dechlorinate first.

Do I need planning permission or a fence?

Rules are local and worth checking before you dig. Many municipalities require fencing, a cover or a safety grid for standing water on a property with young children, and some set a surface-area threshold above which a permit is required. Your household insurer may also want to know.

How long before the pond looks established?

Plants fill in over one growing season. Water chemistry stabilises after roughly six months. Frogs typically colonise in the first or second spring, dragonflies the year after that. Year three is when a pond stops being a project and starts being a place you sit next to.

Everything a backyard pond needs, on one page.