A common method of creating “painted fish” is through 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. This method is 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.