The violet goby, Gobioides broussonnetii, is a member of the family Gobiidae of the order Perciformes. It is native to brackish waters near the Atlantic coast of North and South America. Its commercial trade name is Dragon Goby or Dragon Fish. It is often sold as a "highly aggressive" fish, but violet gobies are actually quite docile, and nearly blind.
The Violet gobies has a long, slender, eel-like body. It has one long fin down the length of the body, both on top and bottom. The teeth are very sharp; however these are used for scraping algae off rocks, not fighting. When kept in good condition, dragon gobies develop an attractive iridescent, silvery blue metallic color with a gold blotch pattern. Violet gobies seen in pet stores tanks are generally 3–5 inches (7.6–12.7 centimetres) long. In the wild, violet gobies can grow to 24 inches (61 centimetres) long. In captivity, they seldom grow past 15 inches (38.1 centimetres). No external differences between the sexes are known, although males are more territorial at spawning times.
Habitat and feeding
Violet gobies live mostly in brackish swamps, streams, and estuaries with a muddy substrate. Violet gobies have very small eyes, and as such are mainly scavengers. Their primary method of obtaining food is by scooping up mouthfuls of gravel and sorting edible material from the substrate, and then spitting out the substrate and swallowing the food particles. They also use their highly specialized teeth to scrape algae off of rocks.
Popularity, availability, and hardiness
This is not a very popular fish among hobbyists, probably due to its poor availability. Dragon gobies disappear from the market for long periods. They are not as hardy as the paradise fish but are hardier than the dwarf gourami.
Behaviour and aggression in captivity
Despite its fierce looks, large mouth, and many teeth, the violet goby is a peaceful scavenger. If well fed, it usually will not bother smaller fish. Any small, peaceful, brackish water-tolerant fish can coexist with violet gobies. Examples include mollies, guppies, swordtails, platies, and tetras. The violet goby is only kept with peaceful fish, as it has poor eyesight and may be bullied by more boisterous fish.
Temperature, pH, and salinity
Violet gobies are scavengers in the wild and need a varied diet for optimum health, but are limited by two factors, their small throat size and very poor vision. Despite having huge tooth-filled mouths, these fish actually have very small throats and this limits the kinds of foods they eat. They eat both animal and plant-based foods. Unless they are in a species specific tank, violet gobies cannot compete with other fish to find food, because of poor eyesight. Violet gobies often feed at night.
They typically spawn in a group of one male and three or more females. Females require many hiding places, while the male needs a site to build his nest. Spawning is triggered by feeding on live meaty foods, and a decrease in salinity from 7 parts per thousand (1.006–1.008sg) to around 5 parts per thousand (1.004), followed by an increase to 30 parts per thousand (close to regular marine salinity, about 1.023sg). The male spawns with several females over the course of a day. After spawning, the male guards the eggs. After 1.5–2 days, the eggs hatch. Fry eat infusoria (particularly rotifers) and "green water" (single celled algae) for up to a month. They then eat baby brine shrimp nauplii.