Dyed and Tattooed Aquarium Fish: Methods, Health Effects and Campaigns

How aquarium fish are artificially coloured by injection, dye baths and laser tattooing, what sources report on mortality and disease, and how retailer pledges and campaigns developed.

What dyed and tattooed fish are

Dyed, painted or tattooed fish are aquarium fish whose colours have been added artificially rather than produced by genetics or diet. The practice differs from selective breeding of colour strains and from fluorescent transgenic fish, whose colour is inherited; those topics are covered in separate genetics guides. Wikipedia notes that artificial colouring is not permanent and usually fades within six to nine months.

Methods

Injection: the fish is injected, often several times, with a hypodermic syringe containing a bright fluorescent dye (Wikipedia). Indian glassfish (Parambassis ranga) are treated this way by injecting dye into their transparent tissue and are then sold as 'disco fish' (Wikipedia).
Dye baths: the fish is first dipped in a caustic solution that strips its outer slime coat and then dipped in dye. Wikipedia reports that dipping and injection both have a very high mortality rate.
Laser tattooing: a low-intensity laser is used together with a dye. Wikipedia notes that this technique was originally developed by scientists for fisheries work.
Dyed food: colour-enhancing foods usually rely on natural pigments such as beta-carotene and are generally safe, but Wikipedia reports that harmful dyes are sometimes used by wholesalers.
Hormones: hormone treatment can intensify colour, but Wikipedia states it can also make female fish infertile.

Practical Fishkeeping has reported that suppliers have never confirmed the exact method used to apply pigment to some newer tattooed fish. Marks have appeared on the flanks, fins and mouth, and in some cases as text or logos on the body.

Species commonly affected

Wikipedia lists Indian glassfish, black tetras, oscars, Corydoras catfish, African rift lake cichlids, suckermouth catfish, blood parrot cichlids and goldfish among the fish commonly dyed. Practical Fishkeeping has also reported coloured white parrot cichlids, the giant gourami (Osphronemus goramy) sold as kaleidoscope or polka-dot gourami, and convict cichlids (Cryptoheros nigrofasciatus). Wikipedia and a Practical Fishkeeping ethics debate both record tattooed hybrid cichlids such as blood parrots and flowerhorns.

Documented mortality and health effects

According to Wikipedia, a 1998 study found signs of Lymphocystis infection in over 40% of painted glassfish, compared with 10% of unpainted glassfish. Wikipedia also states that fish injected with dye often die without obvious external symptoms, presumably from kidney disease. The Wikipedia article on Parambassis ranga adds that the process is not painless, causes kidney damage, and leaves the fish vulnerable to parasites, fin rot and the viral disease lymphocystis. Healthy unpainted glassfish may live three to four years in captivity. Practical Fishkeeping describes artificial colouring as having serious health and welfare implications.

Campaigns and retailer pledges

Wikipedia reports that Practical Fishkeeping started a campaign in 1996 asking retailers to stop selling dyed fish, which led to a significant fall in UK sales. The magazine's earlier research had shown that some 'coloured' or 'painted' glassfish in the trade got their colour from dye injections. After dyed fish became more common again, the magazine relaunched the campaign as a global effort. It asks aquatic retailers, wholesalers and importers to sign a pledge not to stock dyed fish. Shops that sign receive a poster to display and are listed in the magazine and on its website.

According to Practical Fishkeeping, nearly 90% of its readers said they would choose to buy from dealers who pledge not to stock dyed fish.
A later Practical Fishkeeping update reported that over 50% of UK tropical fish shops had signed the pledge.
A further report put the figure at 70% of UK stores, while noting that compliance appeared to be declining.

Identifying dyed fish

Unnatural, sharply defined marks such as thick stripes along the flanks, coloured lips or written shapes indicate tattooing (Practical Fishkeeping).
Bright fluorescent colour inside the transparent body of a glassfish, sold as 'disco fish', indicates dye injection (Wikipedia).
Colour that fades over several months indicates artificial colouring rather than genetic colour (Wikipedia).
Blood parrot cichlids occur naturally in red, yellow and grey; other colours come from injected dye (Wikipedia).