A goldfish pond through the year

The pond guide covers what a goldfish pond is. The building page covers making one. The seasonal feeding page covers what to feed and when.

This is the operational year: what to do, when, with the reasoning linked rather than restated.

The thermometer gives the instruction

Every month named on this page is a proxy. The pond does not know what the calendar says, and a mild February and a cold April happen regularly.

Measure the water, at the depth the fish are using, and let that decide. A floating thermometer reads the surface film, which in winter can be several degrees away from where the fish actually are — the feeding page covers why that matters and how to take a reading worth having.

Where the temperature and the month disagree, the temperature is right.

The pond, by the thermometer

1. below 8–10 °C Deep winter No food (a pond convention). Keep a hole in the ice, and leave them alone.
2. 10–15 °C Spring and autumn Wheatgerm food, a keepers’ habit. In autumn, taper it and net the pond.
3. 20–25 °C Summer The full ration. Aeration, top-ups, growth.
4. above 25 °C A warm spell Feed lightly and early in the day. Shade, aeration, and a check at dawn, when oxygen is lowest.

Early spring: 6 to 12 °C

A delicate part of the year, and one that catches people because it looks like the easy part.

Do not feed yet. Not until the water has been reliably above about 10 °C for several days, and then only sparingly on a wheatgerm food.

Bring the filter back gently. If it was shut down, clean it and restart it before the feeding increases rather than after, so the bacterial colony has time to catch up with the load. The filtration guide covers the biology.

Clear winter debris. Dead plant material, fallen leaves, accumulated sludge — but by lifting rather than by stirring, and not all at once. Disturbing an entire sludge bed in one go releases a great deal at once.

Small water changes, not large ones. A big volume of cold tap water into a pond in March is a temperature event as much as a chemical one.

Watch the fish closely. This is the window when problems appear.

Late spring: 12 to 18 °C

The busy season.

Feeding builds, from wheatgerm every few days up to a normal ration as the water climbs through the teens.

Expect spawning. Chasing, tubercles on the males, eggs on the plants and the liner. The breeding page covers what is happening and what it costs the female — cover for her to rest behind is the practical response, and the eggs will mostly be eaten within hours.

Plants come back. Divide and repot overgrown marginals, thin oxygenators, and top up floating cover.

Blanketweed starts. See below.

Herons are often at their most active, and a heron can empty a small pond in a week. Netting or mesh over the whole pond is the dependable defence; depth and steep sides help but do not remove the risk.

This is when to add fish, if you are adding any — a new fish in May has a whole season to establish. The choosing guide covers the selection and the handling guide the introduction.

Summer: 18 to 25 °C and above

An oxygen season rather than a warm one.

Warm water holds less dissolved oxygen while the fish need more of it, and a pond that is fine in June can be in trouble during a heatwave. The aeration guide covers the mechanism.

Feed in the morning during hot spells, so digestion — which itself consumes oxygen — happens during the day rather than overnight, when oxygen falls.

Reduce the ration in a heatwave, and do not compensate afterwards.

Increase aeration: a fountain, a waterfall, an air stone, anything that moves the surface.

Watch for gasping at the surface at dawn, which is when oxygen is lowest and when a struggling pond shows it.

Top up evaporation with conditioned water, and note that topping up with tap water repeatedly concentrates minerals over a season — which is one of the few places rainwater is genuinely the right choice.

This is when fish grow. Summer feeding in a pond, with insects and crustaceans supplementing what you provide, is where pond goldfish get their size.

Early autumn: 18 down to 12 °C

A consequential season, and one that often gets too little attention.

Feed well. The fish is building the reserves it will live on for months, and the water is still warm enough for it to digest properly. Skimping here produces a thin fish going into winter.

Net the pond before the leaves fall, not after. A net over the surface from early autumn until the trees are bare saves an enormous amount of decomposing organic matter.

Do the year’s real maintenance. Clear sludge, service the filter, cut back and remove dying plant material — all while the water is warm enough for the bacterial colony to recover from being disturbed.

Treat anything that needs treating, now. A fish with an ulcer or a fungal patch does not get better over winter; its immune system is slowing just as it needs it, and most treatments depend on a metabolism that is running. The medications page covers why cold water changes the arithmetic.

Do not add new fish.

Late autumn to early winter: 12 down to 6 °C

Shutting down.

Taper the feeding: normal food in smaller amounts, then wheatgerm, then nothing below about 10 °C.

Keep the netting on until the trees are bare.

Lift or reposition the pump so it is not drawing the slightly warmer deep water up to a freezing surface — or shut the system down, cleaning the filter rather than leaving stagnant water in it.

Bring fancies indoors where winters are cold. Round-bodied breeds are poor candidates for a frozen pond, and the pond guide covers which fish overwinter outdoors well. The tamasaba, a deep-bodied fancy with a single tail from northern Japan, is reported to withstand cold well, though that is a report rather than a measurement and does not carry over to the other fancies.

Check the pond after storms, for debris and for anything blown in.

Winter: below 6 °C

Almost entirely a matter of leaving it alone.

Keep a hole in the ice, so gases can exchange. Melt it with a pan of hot water — never break it, because the shock waves can harm the fish below.

Do not feed.

Do not disturb the fish. A goldfish in winter torpor resting on the bottom is doing what the species does. It is not distress.

Do not do large water changes. A big volume of near-freezing tap water does more harm than the nitrate it removes.

Do look, briefly, from the edge. Not to intervene, but so that you know.

Testing and water changes

Less often than an aquarium, and not never — which is where most pond keepers land wrongly in one direction or the other.

Test through the growing season. Ammonia and nitrite should read zero once the pond is established; nitrate tells you whether the plants and the water changes are keeping up with the feeding. Monthly through spring and summer is plenty for a settled pond, and weekly for a new one. The testing guide covers the readings.

Change water modestly and regularly through the warm months — a fraction of the volume, conditioned, at a similar temperature. A pond buffers far better than a tank, which means the changes can be smaller and less frequent, not that they can be skipped.

Top up evaporation as it happens rather than letting the level fall and then refilling in one go. A large sudden addition of cold tap water is a temperature and chemistry event; a regular small top-up is neither.

And not in winter. Below about 8 °C, leave it. The nitrate is not going anywhere and the fish have no capacity to absorb the change.

Predators, through the year

Herons are a main one, often most active in spring, and they typically wade in at dawn from a shallow edge. A heron can empty a small pond in a week. What is dependable is netting or mesh over the whole pond; depth and steep sides reduce wading access without removing the risk, somewhere the fish can get underneath helps, and perimeter wires and fences have had limited success. Decoys have mixed results, and keepers report that real birds often ignore them.

Cats take fish from shallow water and are a year-round, low-level problem that depth and edge design mostly solve.

Everything else depends on where you are — raccoons, otters, mink, and fish-eating birds other than herons all occur somewhere.

And the diagnostic worth knowing: losses you cannot explain are often a bird. A pond that is quietly one fish lighter every few days, with clean water tests and no sign of disease, is probably being fished. Watch it at dawn for a morning, or look for a flattened patch at a shallow edge.

The first year of a new pond

Different from an established one in ways that catch people, and worth separating out.

It has to cycle, exactly like an aquarium — the cycling guide applies without modification, and a pond filled last weekend is not ready for fish.

It will go green, usually spectacularly, some weeks after filling. A new pond has nutrients and light and nothing established to compete for them, so single-celled algae bloom. It passes as the planting establishes, and it is a problem a UV clarifier does help with — the UV buying guide covers sizing one, where the trap is that the headline volume rating assumes a pond with no fish in it. The cloudy water page covers the distinction from a bacterial bloom.

The blanketweed will be at its worst in the first two summers, for the same reason: nothing else is using the nutrients yet.

The plants will not be doing their job yet. A newly planted pond has a fraction of the nutrient uptake it will have in two years, which is why patience beats intervention.

Stock slowly, a few fish at a time over months rather than all at once, and keep the numbers below what the stocking arithmetic allows while the system matures.

Most of what looks like a failing new pond is a pond that is three months old and behaving exactly as a three-month-old pond does.

The year in one table

Water	Roughly	Feeding	The job
Below 6 °C	Deep winter	None	Keep a hole in the ice; leave them alone
6–10 °C	Early spring, late autumn	None	Spring: restart gently. Autumn: finish the work
10–15 °C	Spring, autumn	Small amounts, wheatgerm by habit	Spring: watch closely. Autumn: taper, net the pond
15–20 °C	Late spring, early autumn	Building, then full	Spring: spawning, plants, herons. Autumn: feed well, do the maintenance
20–25 °C	Summer	Full ration	Aeration, top-ups, growth
Above 25 °C	Heatwave	Reduce; early in the day	Aeration, shade, watch at dawn

Blanketweed

Barely covered anywhere and universally encountered, so it gets its own section.

What it is: filamentous green algae — long green strands that cling to plants, liner and waterfall, and that can be twisted off on a rough stick. It is not the same organism as the single-celled algae that turn water green, and that distinction decides what works against it.

Why you have it: nutrients and light. A pond with fish in it, fed regularly, in the sun, is a nutrient-rich, well-lit environment, and blanketweed is simply the organism best placed to use that. It peaks in late spring and summer.

What works, slowly and durably:

• Reduce the nutrient supply. Feed less, remove sludge, keep leaves out, and do not overstock. This is the actual cause and the actual cure.
• Compete with it. Fast-growing plants — oxygenators, marginals, floating plants — use the same nutrients, and a well-planted pond has far less blanketweed than a bare one.
• Shade the water. Lily pads and floating plants over perhaps a third to a half of the surface remove much of the light it depends on.

What works immediately and does not last:

• Twisting it out on a stick. Effective, satisfying, and it grows back until the nutrients change. Leave what you remove beside the pond for a day so anything living in it can get back.

What does not work:

• A UV clarifier. It damages single-celled algae in suspension so they clump and are filtered out, and keepers report that it does little against filamentous algae attached to surfaces. This is a common disappointment in pond keeping, and it is worth knowing before buying one for this purpose. The algae page covers the same distinction indoors.
• Removing every plant to “starve” it. This removes the competition and makes it worse.

And the one that is genuinely uncertain: barley straw, which is traditional and widely sold, with keepers reporting mixed results. It is cheap, so it is a reasonable thing to try and a poor thing to rely on.

The annual checklist

When	Job
Early spring	Restart filter, clear debris gently, check equipment, start feeding late
Late spring	Divide plants, net against herons, expect spawning, add fish if adding
Summer	Aerate, top up, feed mornings in heat, watch at dawn
Early autumn	Feed well. Net before leaf fall. Clear sludge. Service the filter. Treat anything that needs it.
Late autumn	Taper feeding, lift or stop the pump, bring fancies in
Winter	Keep a hole in the ice. Otherwise nothing

The bolded row is the one that matters most and the one most often skipped. A pond that has a good autumn mostly has a good year.