Once familiar with the face of its owner, swimming towards the fish keeper during feeding time can be observed, and hand-feeding becomes possible.

While this behavior is welcomed by goldfish owners, it may be problematic in outdoor ponds where predators may eat such friendly prey.

After some time, this schooling behavior eventually ceases, and soon every individual fish will swim and explore the aquarium on its own.

If placed in a bare aquarium, goldfish will settle to the bottom and only move when fed or frightened by sudden jolts.

However, if put in a tank with sufficient gravel, aquarium accessories, or plants (real or fake), they will make themselves at home.

Cannibalism is usually rare (occurring maybe when a fish is dying or dead), but in cramped, stressful situations, goldfish may behave unpredictably[citation needed].

If introduced in too great a number before these bacteria grow, the goldfish will die from breathing in too much of their own untreated excrement.

Even after the development of the biological filter, it is necessary to change about 20% of the water at least one time a week, or as necessary to prevent a buildup of harmful nitrate.

Outdoor ponds have similar care requirements as indoor tanks, with some notable exceptions such as amount of sunlight, natural and artificial environment pollution (i.e. dead leaves, debris, runoff), algae, selection of pond mates (i.e. algae eaters, frogs, etc.).

Outdoor ponds tend to become miniature aquatic ecosystems, attracting various animals and plants (see water gardens for more information).

In hot climates, it is important that pond temperatures do not rise to dangerously high levels that will kill the fish.

Mature goldfish will require a minimum tank size of 20 gallons per fish to allow ample swimming room and create a "buffer zone" that will delay the effects toxins like ammonia and nitrites from building up quickly.

Filtration systems help keep the tank clean and remove toxins, while also promoting good bacteria growth.

Cleaning the filter sponges and vacuuming the gravel can help remove debris and rotten food.

It is important to avoid using tap water to clean decorations, filters, and equipment as it can kill good bacteria.

Goldfish require adequate exposure to light to promote healthy sleeping patterns and maintain a similar environment to their natural habitat.

In breeding conditions the male develops small white spots on his gill covers and the female will become plump.

Goldfish are notoriously dirty, producing much waste, and continually stir up the substrate in their infinite search for food.

In small aquariums, illnesses in common goldfish can quickly become fatal and require prompt treatment.

[4] Swim bladder disease may be caused by insufficient fiber, poor water quality, or overfeeding.

Treatment starts with testing the water to ensure cleanliness, then the goldfish should fast for three days if no improvements are made.

Fin rot must be treated early to prevent further damage to the body tissue, so the disease does not spread.

[4] White Spot Disease is caused by the protozoan parasite Ichthyophthirius multifiliis, colloquially known as "ich."

This causes the epidermal cells to proliferate and form pinhead-sized white spots that appear on the fins and body.

If left untreated, in a small volume of water such as an aquarium the infected fish can quickly succumb to the rapidly multiplying parasites.

The fish is left in the salt bath for a few more days to allow its skin to heal while the water temperature is slowly decreased.

[6] Treatments start by moving the goldfish to a hospital tank and then clearing the environment with Methylene blue.

Goldfish can live for long periods of time if they are fed a varied diet and housed in proper water conditions.

Common goldfish are often used as live prey for freshwater turtles, notably red-eared sliders, and carnivorous fish such as oscars.