Neon tetra disease is difficult to diagnose reliably and even more difficult to treat. Despite the name, neon tetra disease can affect a range of other tetras besides neon tetras and has been reported from a variety of other aquarium fish as well.
At very low levels of infection, there may be no symptoms visible at all, and it is usually the case that the more obvious symptoms of neon tetra disease are only apparent on heavily infected aquarium fish a few days away from death. Infected aquarium fish will often spend less time with their schoolmates than normal, typically hiding away under aquarium plants and showing no interest in fish food. The aquarium fish’s colors fade, and sometimes gray or white patches on the flanks become apparent. In advanced cases, the aquarium fish may have trouble swimming, and the fish may develop odd swellings or contortions indicative of damage to the musculature. Usually, the aquarium fish dies within two or three days of the first symptoms of neon tetra disease becoming apparent.
Unfortunately for the aquarist, several other diseases can cause similar symptoms to neon tetra disease, including systemic bacterial infections, chronically poor diet and/or environmental conditions, and even old age. Some aquarists have coined the term “false neon tetra disease” to refer to bacterial infections with broadly similar symptoms.
The parasite responsible for neon tetra disease is called Pleistophora hyphessobryconis. It is a single-celled organism that gets into its host by being consumed alongside fish food. The parasite was first identified in neon tetras, hence the common name, but it has also been reported from a range of other tetras as well. Less frequently, it has been reported to affect non-tetras including minnows, danios, goldfish, and even angelfish.
Pleistophora hyphessobryconis gets into the aquarium fish through accidental ingestion. Under aquarium conditions, this is most commonly some sort of scavenging or cannibalism, the parasites inside a dead fish being consumed by a healthy fish. Once inside the gut, the parasite spreads throughout the fish’s body and eventually gets into the skeletal muscles where it matures. Eventually, the parasites produce large numbers of spores, and when the host fish is eaten by a predator or consumed by a scavenger after death, the spores are then able to get into new fish.
Parasites in tissues other than the muscles, for example, those in the gut or kidneys, may release viable spores into the aquarium water in other ways as well, even before the host fish dies. This is why isolating infected aquarium fish is so important. In one laboratory experiment, an infected neon tetra was placed alongside a group of ten zebra danios, and no fewer than seven of those danios became infected!
There are no effective medications for neon tetra disease, and infected fish are normally removed and euthanized at once to prevent subsequent infection of healthy fish. However, because neon tetra disease can be confused with bacterial infections, some aquarists may want to place infected fish in a quarantine aquarium and treat them with antibiotics. If the aquarium fish recovers, then it can be returned to the main aquarium after a suitable quarantine period.
Toltrazuril has been used experimentally to treat neon tetra disease microsporidian parasites including Pleistophora hyphessobryconis, but thus far there are no medications commercially available.
There is some evidence that healthy aquarium fish can fend off neon tetra disease infections under their own steam, and conversely, neon tetra disease seems to be most common when aquarium fish are stressed or otherwise weakened. In the case of neon tetras for example, their maintenance in hard water and at excessively high temperatures are surely reasons why so many people have problems keeping them alive for anything like their full lifespan of 4 to 5 years. While they can tolerate harder water for short periods, for long-term success they should be maintained at 2 to 10 degrees dH, pH 6 to 7.5. Water temperature should also be carefully controlled, ideally between 72 to 75 degrees F, making neon tetras particularly suitable for maintenance alongside other aquarium fish that prefer relatively cool conditions, such as pearl danios, black phantom tetras, golden dwarf barbs (Puntius gelius) and Corydoras catfish.
Because neon tetra disease is highly contagious it is very important to remove affected aquarium fish as soon as possible. Treatment of neon tetra disease with an antibiotic may be worthwhile if the fish is still mobile and feeding, but otherwise the fish should be euthanized and the body disposed of in the trash. Needless to say, neon tetras should never be purchased from aquariums with sick or ailing fish, and even when apparently healthy fish are bought, they should be quarantined for a period of 4 to 6 weeks before being placed in a community aquarium.
Some aquarists have found it difficult to keep neon tetras alive for more than a few months, often losing one or two every few weeks until their initially beautiful school of neon tetras is completely gone. One workaround here is to avoid neon tetras in favor of similar fish that aren’t affected by neon tetra disease, or if they are, less commonly. Cardinal tetras for example are mostly wild-caught and appear to be plagued less by neon tetra disease than farmed neon tetras. Cardinal tetras are a bit more expensive of course, and must be kept in warm, soft, acidic water conditions; aim for 79 to 82 degrees F, 2 to 10 degrees dH, pH 5.5 to 7.0. In harder water Celebes rainbowfish (Marosatherina ladigesi) and dwarf rainbowfish (Melanotaenia praecox) make good alternatives, and both have the luminous electric blue color that makes neon tetras so eye-catching.