The neon tetra (Paracheirodon innesi) is a freshwater fish of the characin family (family Characidae) of order Characiformes. The type species of its genus, it is native to blackwater or clearwater streams in southeastern Colombia, eastern Peru, and western Brazil, including the tributaries of the Solimões where the water is between 20–26°C. It is not found in the whitewater rivers of Andean origin. Its bright colouring makes the fish visible to conspecifics in the dark blackwater streams, and is also the main reason for its popularity as a tropical fish.
The Neon Tetra has a light-blue back over a silver-white abdomen. The fish is characterized by an iridescent blue horizontal stripe along each side of the fish from its nose to the base of the adipose fin, and an iridescent red stripe that begins at the middle of the body and extends posteriorly to the base of the caudal fin. Most, if not all, will develop a olive green sheen lining their backs. The fish is completely transparent (including fins) except for these markings. During the night,the blue and red become silver as the fish rests—it reactivates once it becomes active in the morning. It grows to approximately 3 cm (1.25 in) in overall length. Sexual dimorphism is slight, the female having a slightly larger belly.
The Neon Tetra was first imported from South America and was described by renowned ichthyologist Dr. George S. Myers in 1936, and named after Dr. William T. Innes. P. innesi is one of the most popular aquarium fish, having been bred in tremendous numbers for the trade. Most neon tetras available in the United States are imported from Hong Kong, Singapore, and Thailand, where they are farm raised, or to a lesser extent (<5%) from Colombia, Peru, and Brazil, where they are collected from the wild. During a single month, an average of 1.8 million neon tetras with an estimated value of $175,000 are imported into the United States for the aquarium trade. With the exception of home aquarists and a few commercial farms that breed neon tetras experimentally, captive breeding on a commercial scale is nonexistent in the United States.
While commercially bred neon tetras have adapted well to a wide range of water conditions, in the wild they inhabit very soft, acidic waters that are usually cooler than the 25°C most tropical aquaria are maintained at. Neon Tetras have a lifespan of about five years.
Neon tetras are considered easy to keep in a community aquarium that is at least 60 cm (24 inches), with a pH of 5.0–7.0 and KH of 1.0–2.0. However, they will not tolerate dramatic changes to their environment. They tend to be timid and, because of their small size, should not be kept with large or aggressive fish who may bully or simply eat them. Fish that mix well in an aquarium are other types of tetras, such as the rummy-nose tetra, cardinal tetra, and glowlight tetra, and other community fish that live well in an ideal Tetra water condition. They can also be with bettas (siamese fighting fish) including the male. Mid-level feeders, they are best kept in schools of five to eight or more, for the "shoaling" effect when they move around the tank. They shoal naturally in the wild and are thus happier, more brightly colored, and more active when kept as a shoal as opposed to singly. Their colour and the iridescent stripe may become dim at night, and can be virtually invisible after a period of darkness. The color may also fade during a period of stress, such as human intervention into the tank. Neons are best kept in a densely planted tank with subdued light and an ideal temperature of 78-80 degrees Fahrenheit to resemble their native Amazon environment.
Neon tetras are omnivores and will accept most flake foods, but should also have some small foods such as brine shrimp, daphnia, freeze-dried bloodworms, tubifex, and micro pellet food to supplement their diet. A tropical sinking pellet is ideal as most brands of these include natural color enhancers that really bring out the color in neon tetras.
The male is slender, and the blue line is straighter. The female is rounder, producing a bent blue line. Some say that the females look plumper when viewed from above but this is disputed. However, the 'straightness' of the line and the plumpness of the female might occasionally be due to the eggs she is carrying.
To breed Neon Tetras, place a pair of the species in a breeding tank without any light, and gradually increase the lighting until spawning occurs. Other inducers include mosquito larvae and a hardness of less than 4 degrees. Some also recommend letting the level of nitrates rise, then do at least 50% water change to simulate the fresh rain the tetras get in their natural habitat, the Amazon. It is recommended that everything you place in the aquarium be sterilized, as well as the aquarium top. Because the adults will often eat newly-hatched fry, it is best to remove them as soon as the eggs have been laid. The eggs are especially sensitive to light. Eggs will hatch within 24 hours of the laying. Fry can be fed rotifers, especially infusoria and egg yolk for 1 to 4 weeks, followed by nauplii of brine shrimp, shaved cattle liver, and formulated diets. Fry will achieve their adult coloration at approximately one month of age. Adults can spawn every two weeks.
Unfortunately, neon tetras are occasionally afflicted by the so-called "Neon Tetra Disease" (NTD) or Pleistophora which is usually fatal to the fish, and currently without a cure. This sporozoan disease is caused by Pleistophora hyphessobryconis.
The disease cycle begins when microsporidian parasite spores enter the fish after it consumes infected material such as the bodies of a dead fish, or live food such as tubifex, which may serve as intermediate hosts. The disease is most likely to be passed on from newly acquired fish, which have not been quarantined.
Note that there is a so-called "false neon disease", which is bacterial, and shows very similar symptoms. It is impossible for the home aquarist to determine for certain the difference between NTD and false NTD on the basis of visible symptoms alone, without laboratory backup. This disease has also been confused with Columnaris (mouth rot, mouth fungus, 'flex').
To date, there is no known cure: the only 'treatment' is the immediate removal of diseased fish to preserve the remaining fish, but no way to save the diseased fish. The use of a diatom filter, which can reduce the number of free parasites in the water, may help.
The green neon tetra (P. simulans) and black neon tetra (Hyphessobrycon herbertaxelrodi) are distinct species—the latter belong to an altogether different genus—and not color varieties. The cardinal tetra (P. axelrodi)—also sometimes called the red neon—is a very similar species, and is often confused with the true neon tetra. In a domestic aquarium, the two species will school together. It is distinguished by a lateral red stripe that runs the length of its body. Hyphessobrycon innesi, on the other hand, is an obsolete synonym for P. innesi, the neon tetra itself.