Goldfish in a tub, trough or patio pond

A goldfish in a sunny tub on a patio is one of the pleasures of keeping them. Daylight, algae to graze and insects to chase do things for a goldfish that a tank struggles to match, and many fish that spend a summer outdoors come back in autumn larger and more richly coloured than they went out.

It is also one of the easier ways to lose a fish, because a container looks like a small pond and behaves like nothing of the kind. This page is about the difference, and about doing it in the way that gives a fish a good season rather than a dangerous one. The pond guide sets out the short version; this is the long one.

A summer home, not a small pond

Everything that makes a pond forgiving comes from volume and surface area, and a container has little of either.

That is why the same fish behaves so differently in each:

	Container of a few hundred litres	Garden pond, deep enough for its winters	Indoor tank
Summer afternoon	Warms fast; can get very warm in sun, with less oxygen	Barely moves	Stable
Oxygen on a warm night	Can run short without air	Usually fine	Stable with a filter
Winter	Can freeze nearly solid	An unfrozen layer under the ice	Stable
Right for	A summer season	Single-tails, year round	Any goldfish, year round

The practical conclusion follows directly. Wherever winters bring hard frost, a container pond is a summer arrangement. In a mild climate a large container can work all year, but even there it needs shade, aeration and close attention in heatwaves.

How big

The largest container you can manage. A few hundred litres behaves far better than eighty, and nothing in the rest of this page matters as much.

Wide beats deep. The surface is where oxygen comes in, so a wide, shallow trough gives more margin on a hot night than a narrow, deep pot of the same volume. Depth still helps with heat, so the ideal is broad and reasonably deep.

Measure it rather than trusting the label. Containers are sold by nominal volume, and the water they actually hold, filled to a sensible level below the rim, is noticeably less. Work it out from the inside dimensions.

What it is made of matters as much as its size:

• Food-grade plastic — stock tanks, troughs and preformed pond liners, and mortar tubs where the maker says they are food-grade — is the straightforward choice.
• A wooden barrel needs a liner. Old wine and spirit barrels leach residues, and the wood swells, shrinks and leaks.
• Glazed ceramic is fine if the glaze is intact and the drainage hole is sealed properly.
• Galvanised metal should be lined or avoided. The zinc coating can leach into the water, particularly soft water, and fish are sensitive to it.

Where to put it

Morning sun, afternoon shade. The afternoon sun is what overheats a container, and a position that loses the sun by early afternoon avoids most of the danger before anything else is needed.

On a level, solid base. A litre of water weighs a kilogram, so a 300-litre container weighs more than 300 kilograms with its water, stones and plants. That is a real load for a balcony or a raised deck, and the floor loading guide covers how to check.

Away from overhanging trees and roof water. Falling leaves rot in a small volume and discolour it, and a downpipe or a roof edge can flood the container with run-off in a storm.

Near an outdoor socket, if you will run a pump or an air pump, on a protected circuit and with a drip loop in the cable. The pond building guide covers outdoor electrics, which are worth doing properly rather than conveniently.

Shade, air and water movement

In a container, heat is the summer danger, and cold only becomes one in autumn.

• Shade part of the surface. A water lily, floating plants or simply a shade cloth over part of the container. Leave much of the surface open, as a pond does, so the water can still breathe.
• Aerate on warm days and warm nights. An air pump with an air stone, a small fountain or a solar pump all work. Oxygen falls as the water warms, and it falls further at night when plants and algae stop producing it and start using it; the aeration guide covers why.
• Filter it if you can. A small pond filter or an air-driven sponge filter is a genuine improvement, and an air pump that runs a sponge filter aerates and filters at once. The air pump guide covers sizing.
• Keep a thermometer in it, and read it in the afternoon in hot weather. Water in the high twenties Celsius holds less oxygen and leaves a small container little margin, which is why the temperature guide treats 27 °C as the point to add aeration. Goldfish tolerate such water in laboratory studies, and stress shows by about 34 °C, but a fish gasping at the surface on a hot afternoon needs shade, air and a partial water change with cooler, conditioned water that day.

Which fish, and how many

One or two small fish, not a group. Small volumes are unforgiving: waste builds quickly, oxygen falls quickly on hot days, and there is little margin for a missed water change. The fish will also grow over a summer outdoors, so stock for September rather than May; the stocking guide covers the arithmetic.

Single-tails suit it best. Commons, comets and shubunkins are hardy, strong-swimming and tolerant of cooler water, which is everything a container asks for.

Some fancies can spend a shaded summer outdoors. A fantail in calm, stable water, for example. Round-bodied, slow breeds such as ranchus, telescopes and pearlscales, and breeds with restricted vision such as bubble eyes, are better indoors: keepers report that they cope badly with temperature swings, cannot get away from predators, and have no margin to spare. Never mix fancies with single-tails outdoors, where the competition for food is worse than in any tank, as the tankmates guide explains.

The season: when to put fish out, and when to bring them in

Set the container up before the fish. Fill it, run the filter and the air for a couple of weeks, and seed the filter with media from your indoor tank if you can, so the fish do not arrive in water with no working bacteria. The cycling guide covers what that seeding does.

Put fish out once the water holds steady within their range. Read the thermometer morning and afternoon for a week. When the nights no longer pull the water down out of the range for that fish, and the afternoons do not push it up, the season has started.

Bring them in before it ends. That means before the first frost, and before the water drops towards 10 °C (50 °F), which is also where you stop feeding. Do not wait for the first cold snap, because a container can lose a lot of heat in one clear night.

Match the temperatures at every move. Moving a fish between two very different temperatures in one step is the mistake to avoid. In autumn, lower the indoor tank to the container’s temperature before the move, then bring it back up by no more than a degree or two a day. In spring, do the same in reverse. The handling guide covers moving the fish itself.

Look the fish over when they come in. A summer outdoors exposes them to things an aquarium never does, including birds and wildlife. Keep them in quarantine for at least four weeks before they rejoin other fish, and the anchor worm and fish lice page covers the visible parasites that outdoor fish occasionally pick up.

Feeding outdoors

Feed less than indoors. A sunlit container grows algae and attracts insects, and the fish graze and hunt through the day. Small meals once or twice a day, cleared within a couple of minutes, are plenty in summer.

Follow the temperature, not the calendar. Feed normally in warm water, move to smaller amounts of easily digested food as it cools, and stop below about 10 °C. The seasonal feeding guide gives the bands, which apply to a container exactly as they do to a pond, only faster.

Feed in the morning or early afternoon, not in the evening, so digestion happens while the water is warm and oxygen is plentiful.

Plants for a container

Plants do more work in a container than in a tank: they shade the water, take up nitrate and give the fish cover. Goldfish will nibble and uproot them, so plant in baskets and weigh the surface down with pebbles far too large to fit in a mouth.

• A small water lily gives shade exactly where it is needed. Choose a dwarf variety suited to shallow water, and stand its basket on bricks so the leaves reach the surface.
• Marginal plants in baskets on bricks around the edge add shade and take up nutrients, the planted shelf of a pond in miniature.
• Floating plants shade the surface, but goldfish graze soft ones such as duckweed, frogbit and water lettuce, so treat them as a renewable crop that may need replacing rather than as permanent cover.
• Check what you buy. Some floating plants sold for ponds are banned in parts of the world as invasive species. Buy from a reputable source, and never let a plant, or water from the container, reach a stream or a pond.

The plants guide covers which plants survive goldfish indoors, and most of its reasoning holds outside.

Rain, evaporation and water changes

Heavy rain can overflow a container and wash small fish out with it. An overflow notch covered with fine mesh, or simply keeping the level a few centimetres below the rim, prevents that.

Rain also changes the water. Rainwater is soft and slightly acidic, and a heavy downpour into a small volume can shift the chemistry, which is one more reason to test after a storm; the pH, KH and GH page explains why stability matters more than the numbers.

Top up evaporation with conditioned water, never straight from a hose. And keep doing partial water changes: a container has no more capacity to dilute waste than a tank of the same volume, and a sunny one has less.

A weekly routine

A container changes faster than a tank, so the routine is the same one run a little more attentively.

• Read the thermometer, in the afternoon during hot spells, when it matters.
• Test the water. Ammonia, nitrite and nitrate, weekly; a small volume can go from fine to not fine between one weekend and the next. The testing guide explains what each number means.
• Change a quarter to a third of the water, conditioned and matched to the container’s temperature, and more in a heatwave, when cooler water also takes the edge off the afternoon.
• Rinse the filter sponge in container water when the flow drops, never under the tap.
• Check the cover, the overflow and the air line, which are the three things that fail quietly outdoors.
• Watch one whole feed. Every fish should be eating; one that hangs back is the first sign of trouble in a container just as in a tank.

Going away in summer

A container is the hardest thing to leave in summer, because heat and oxygen can turn in a single day. Before a holiday:

• Add shade and keep the air running around the clock.
• Do a water change a day or two before you go, not on the morning you leave, so any problem shows while you are still there.
• Do not leave extra food. A healthy adult goldfish can generally go a few days without being fed, and fewer in warm water, and in a sunny container it will graze algae and insects in the meantime; the fasting page covers the limits. Uneaten food fouls a small volume of water quickly.
• Ask someone to look in during a heatwave, check the thermometer and the cover, and top up with conditioned water if the level falls.

The holiday guide covers the rest, and its point holds outdoors as well as in: the fasting limits vets give are for emergencies and not a plan, so for anything longer than a few days arrange a briefed sitter.

Green water, algae and mosquitoes

Green water is common in a sunlit container and is not in itself harmful: the fish graze it, and it shades the water. Its costs are that you cannot see the fish, and that the algae consume oxygen at night, which is when a warm container is already at its lowest. Aeration takes care of the second. The algae guide covers the rest.

Goldfish eat mosquito larvae eagerly, so a stocked container rarely produces mosquitoes, while an empty container of standing water is an ideal nursery for them. Keep it stocked, or empty it. And never treat that as a reason to put goldfish into ponds, ditches or waterways; the rehoming guide covers why releasing them does real damage.

Predators, and covering it

A container is an easier target than a pond: the water is shallow, the sides are steep and there is nowhere to hide.

• Cats fish from the rim.
• Herons and other birds take fish from open water, and a heron can empty a small container in one visit.
• Some regions have raccoons, foxes or other mammals that learn quickly where fish are.

A mesh cover is the dependable answer, fitted so it cannot be pushed aside, and it also stops leaves falling in. Give the fish somewhere to hide underneath, such as a length of wide pipe or a dense plant, so a startled fish is not simply exposed.

The indoor version

Much of what makes a container appealing — volume for the money, a view from above, a drain plug that makes water changes easy — works indoors too, without the weather. Stock tanks, and mortar tubs sold as food-grade, can be a good home for a large single-tail in a house, and the tank buying guide covers what to look for. That same tub can go outside for the summer and come back in for the winter, with the fish, which is the best of both.