Skeletal deformities are most commonly seen when fish have been inbred, though poor housing and diet can also cause them. Although not treatable, aquarists will need to decide whether deformed fish are suffering, and if necessary, euthanize them.
Deformities of various kinds may be encountered, but the most common are probably bent spines, stunting and incomplete fins. Less common examples include fish with poorly developed swim bladders (“belly sliders”), incomplete gill covers and conjoined twins. Aquarists will usually avoid buying deformed fish, though very young fish may develop deformities if they are not kept properly. On the other hand, aquarists who breed fish will often see deformed fish among the fry.
Of course some deformities may be desirable if the aim is to produce a new variety of a particular fish species. Fancy goldfish are all deformed, as are things like balloon mollies, blood parrot cichlids and veil-tail angelfish. From an animal welfare perspective, a deformity becomes a health issue only when the fish in question cannot feed, swim, or interact with its own kind properly. Even if a deformed fish is enjoying a happy life in your aquarium, you may decide not to breed from that fish, so that its faulty genes are not passed on to the next generation.
Most deformities are caused by one or more faulty genes. In the wild these handicap the fish in question, so it either starves or gets eaten long before it has a chance to breed. But under aquarium conditions handicapped fish are given all the food they need and are kept secure from predators, making it much more likely they’ll survive to sexual maturity. This is why deformed fish are so much more common in ponds and aquaria than they are in the wild.
Wise aquarists will ensure that deformed fish don’t breed by either removing them from their own kind or else destroying any eggs or fry produced. Isolating deformed tetras, barbs, danios and other egg-scatterers isn’t usually necessary because any eggs they produce in a community tank will probably be eaten very quickly. But egg-guarders (such as cichlids) and livebearers are often able to breed successfully under community tank conditions. Deformed specimens of such species are best isolated or, if that isn’t viable, destroyed humanely.
Because inbreeding prevents a healthy shuffling of genes among individuals, it is inbred fish that are mostly likely to produce deformed offspring. Batches of young cichlids and livebearers taken to pet shops by local breeders will often be siblings, and breeding from these has an especially high likelihood of resulting in deformed fry. Wherever possible, breeders should make every effort to obtain parents that aren’t closely related, for example by buying wild-caught fish or by obtaining a male fish from one shop and a female fish from another.
Rearing young fish in tanks that are too small for them is another common reason why fish become deformed. Again, fish breeders are the ones most likely to come across fish deformed this way. As fry mature they need to be maintained in good quality conditions, and this will likely involve regular water changes to reduce nitrate levels in the water as well as upgrading the tank as the fish get bigger.
A poor diet can cause developmental problems, particularly among juvenile fish. Predatory fish are very sensitive to this problem because their diet can be difficult to keep properly balanced. Thiaminase for example breaks down Vitamin B1 and is abundant in crustaceans, some mollusks (e.g., mussels), and some fish meats (e.g., goldfish and minnows). If you’re keeping a predatory fish, it is crucially important to minimize thiaminase intake by favoring meaty fish foods that lack thiaminase, for example tilapia, pollack, cockles and earthworms.
Omnivorous and herbivorous fish are usually easily satisfied with mixtures of flake, pelleted and fresh foods. Green foods are often very rich in vitamins and minerals, especially things like sushi nori, spinach, peas and carrots. But do be aware that flake and pellet foods have an expiration date, and once opened, may lose much of their vitamin content within a couple of months.
Occasionally healthy adult fish develop bent spines and other abnormalities over a period of days or weeks. This may be an indication of an infection of some sort, including Mycobacteria and Pleistophora infections. On the assumption that such infections are likely to be contagious, such fish should be isolated and/or euthanized (Mycobacteria infections for example are invariably fatal and essentially untreatable).
It is not uncommon for very old fish to show slight developmental abnormalities. These include slightly kinked spines, asymmetrical swelling of the abdomen, tatty fins, and loss of scales or color. The point at which a fish becomes “old” obviously varies from species to species. Obviously there’s no cure for old age, though it should be stated that elderly fish often remain surprisingly active even if they do look a little moth-eaten!
Vets will normally euthanize fish using an overdose of an anesthetic called MS-222. Aquarists are unlikely to have access to this, but can painlessly destroy deformed fish (including fry) using clove oil, also know as eugenol. Add 30 drops of this to a container holding two pints (1 liter) of water taken from the fish tank. Stir well, and then immerse the fish using a soft net. Hold down the fish if it likely to jump out or able to breathe air. The fish will become sedated within seconds, and after 10 to 15 minutes will die from suffocation as its breathing rate slows down to zero. Death is normally taken 10 minutes after the last gill movements. Euthanasia>>