Painted fish are ornamental aquarium fish which have been artificially coloured to appeal to consumers. This artificial colouring, also known as juicing, is achieved by a number of methods, such as injecting the fish with a hypodermic syringe containing bright fluorescent colour dye, dipping the fish into a dye solution, or feeding the fish dyed food.

This controversial process is usually done to make the fish a brighter colour and more attractive to consumers. The colouring of the fish is not permanent, and usually fades away in six to nine months. This practice is distinct from the creation of genetically modified fish, whose colouring is passed on genetically and is permanent.

There are a number of methods for introducing artificial colour into fish.

A common method of creating "painted fish" is through dye injection via syringe. Generally, fish are injected multiple times. Fish may also be dipped in a caustic solution to strip their outer slime coat, then dipped in dye. These methods are reported to have a very high mortality rate.

Many varieties of "colour-enhancing" foods for aquarium fishes are available to the consumer. Generally, these foods contain natural dyes, such as beta-carotene, and are not harmful to fish, although, as with other dye methods, the effect is temporary. One source reports that harmful dyes are sometimes used by wholesalers, however.

Fish can also be tattooed using a low-intensity laser with a dye, a process that was developed by scientists for fisheries but is now applied to ornamental fish.

Hormone administration can sometimes increase colouration, although it can also render female fish infertile.

Introduction of genes for fluorescent pigments, derived from corals and jellyfish, results in permanent colouration that is also passed on to offspring, without the need to inject or physically modify the fish themselves.