My longfin goldfish

The goldfish is classified as a coldwater fish , and can live in unheated aquaria at a temperature comfortable for humans. However, rapid changes in temperature (for example in an office building in winter when the heat is turned off at night) can kill them, especially if the tank is small. Care must also be taken when adding water, as the new water may be of a different temperature. Temperatures under about 10 °C (50 °F) are dangerous to fancy varieties, though commons and comets can survive slightly lower temperatures. Extremely high temperatures (over 30 °C (86 °F) can also harm goldfish. However, higher temperatures may help fight protozoan infestations by accelerating the parasite 's life-cycle—thus eliminating it more quickly. The optimum temperature for goldfish is between 20 °C (68 °F) and 22 °C (72 °F). [32]
Like all fish, goldfish do not like to be petted. In fact, touching a goldfish can endanger its health, because it can cause the protective slime coat to be damaged or removed, exposing the fish’s skin to infection from bacteria or water-born parasites. However, goldfish respond to people by surfacing at feeding time, and can be trained or acclimated to taking pellets or flakes from human fingers. The reputation of goldfish dying quickly is often due to poor care. [33] The lifespan of goldfish in captivity can extend beyond 10 years.[34]
If left in the dark for a period of time, goldfish gradually change colour until they are almost gray. [citation needed] Goldfish produce pigment in response to light, in a similar manner to how human skin becomes tanned in the sun. Fish have cells called chromatophores that produce pigments which reflect light, and give the fish colouration. The colour of a goldfish is determined by which pigments are in the cells, how many pigment molecules there are, and whether the pigment is grouped inside the cell or is spaced throughout the cytoplasm.
Because goldfish eat live plants, their presence in a planted aquarium can be problematic. Only a few aquarium plant species for example
Cryptocoryne and Anubias, can survive around goldfish, but they require special attention so that they are not uprooted. Plastic plants are more durable.

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