Goldfish
A carp bred in China for over a thousand years. It has no stomach, it grows to the water it is given, and a well kept one lives past twenty.
Overview
The goldfish is a cold water carp, and it gets far larger and lives far longer than its reputation suggests. FishBase records a maximum of 48 cm and 1.6 kg, and a maximum reported age of 41 years.
It is also the fish most often kept wrong. A goldfish in a bowl is a large, long lived, heavily feeding pond fish in a container with no filter and no room, and almost every problem people have with goldfish traces back to that one decision rather than to the fish.
Temperament
Social, greedy and more responsive to people than most aquarium fish. Goldfish learn feeding times, follow a hand along the glass and recognise the routine of the room.
They do best in groups of five or more. Nothing about them is aggressive, though they will eat anything small enough to fit, including their own eggs and any fish that can be swallowed. Fancy varieties with trailing fins and compressed bodies swim poorly and get outcompeted at feeding time by the flat bodied kinds, so the two are usually kept apart.
Care and grooming
Plankton, benthic invertebrates, plant matter and detritus in the wild, so sinking pellets and blanched greens in a tank. Floating food makes them gulp air at the surface, and in the round bodied fancy varieties that contributes to buoyancy trouble.
No heater. They are a temperate fish, recorded across an extraordinary range in the wild, and room temperature is right. What they do need is filtration and water changes, because a goldfish produces a great deal of waste for its size.
Living and exercise
FishBase sets the minimum aquarium at 100 cm, and a pond suits them better than any tank does. Length and surface area matter more than depth.
Outdoors they overwinter under ice in a pond deep enough not to freeze through, going quiet and eating almost nothing until the water warms. Indoors they need a filter rated well above the tank volume, and a stocking level that assumes a fish reaching a foot or more rather than the three inches it arrived at.
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