Diseases, Infestations, Parasites, and Other Hangers-On

There are literally hundreds of possible maladies that can afflict fish. Some are specific to certain species, and some can easily be transferred between species. Not all are common in the average home aquarium. The causes of common aquarium ailments may be bacteria, viruses, fungi, or parasites.

The following provides a general overview of those diseases you are most likely to encounter in your aquarium. For a more complete listing of tropical fish diseases and their treatments, consult the references in chapter 6.

General Ailments

  • Constipation or Indigestion (not contagious)

    A fish that is constipated or suffering from indigestion is often very inactive and usually rests on the bottom of the tank. In addition, its abdomen generally swells or bulges. This can be caused by an incorrect diet, food that doesn't agree with the fish, or overfeeding. You will need to change the food you are feeding the fish. Some experts add one teaspoon of epsom salts for each five gallons of water in the hospital tank. Starve the fish for three to five days until it returns to being active. When it resumes normal behavior, feed it live or freeze-dried food for one whole week. After one week, return the fish to its normal tank. This is a problem that tends to recur, so make it a point to watch this fish.

  • Swim Bladder Problems (not contagious)

    This is an easy one to diagnose. The fish can't swim properly. It suffers from a loss of balance, swimming on its sides or upside-down, or sometimes somersaulting through the water. Swim bladder disease can result from constipation; from bruising of the swim bladder during handling, fighting, or breeding; or from bacterial infection associated with poor water quality. These problems have been known to correct themselves as the bruised area heals, but you can't always count on this. If you suspect a bacterial infection, improve water quality and treat the fish with a broad-spectrum antibiotic. If this problem is associated with constipation, your fish is more likely to experience a recurrence. Feed your fish something else, as diet can be one of the biggest reasons this disease develops at all.

  • Dropsy or Kidney Bloat (may be contagious)

    This is also known as “pinecone” disease because the belly bloats noticeably and the scales stick out like a pinecone. In general, this disease causes the body to swell due to a buildup of fluid in the tissues. This disease is thought to be caused by water quality problems or some kind of organ failure. Fish generally don't live more than a week after full-blown dropsy makes itself known. Like constipation and swim bladder disease, fish that survive dropsy tend to have recurring attacks. While dropsy is not thought to be contagious, it is best to remove the fish at once. The tank should receive an emergency cleaning.

    Many experts still feel that dropsy is not treatable and that the fish should be immediately removed and painlessly destroyed. Others feel that medicated food is one way to treat dropsy. Still others suggest mixing Furanace with water, 250 milligrams to the gallon. This bath should last only one hour and should not be repeated more than three times in three days. It is thought that Furanace can be absorbed by the fish through the skin. If you choose not to use this remedy, you can always try the old-fashioned salt bath (see sidebar). If your fish does not respond to treatment in two or three days, it should probably be destroyed.

  • Tumors (usually not contagious)

    Obvious lumps, bumps, or protrusions, tumors sometimes look like a large blister or wart. They have been known to grow to the size of a large screw head. They can be surgically removed, but only by a veterinarian.

  • Pop-Eye (Exophthalmus; not contagious)

    This disease causes the eyes to bulge from their sockets and is, therefore, easy to recognize for most tropical fish. The condition is generally caused by poor water quality and the subsequent chronic stress. Recovery may take several days if efforts are made to improve the water quality. Some feel that food should be withheld for two or three days until tank conditions are corrected.

    Marcus Welby Meets Flipper

    Many veterinarians see people whose patients are fish. To find out who takes fish on as patients near you, call your local veterinarian. He or she will usually refer you to another vet who specializes in aquatic life without too much difficulty.

Bacterial, Viral, and Fungal Infections

  • Furunculosis (Ulcer Disease; contagious)

    This bacterial infection will go for some time unnoticed, but then it will spread rapidly. These bacteria infect the flesh under the scales somewhat like skin flukes, which we'll cover later. However, this infection is first manifested by the appearance of bumps underneath the scales. A short time later, the bumps rupture and create large bleeding ulcers. That is why this ailment is sometimes referred to as “ulcer disease.” There is no certain cure for this.

    While some fish have actually survived, large scars resulting from the infection often prove a problem for them. Fish with these kinds of ulcers should probably be destroyed. The remaining fish should be treated with tetracycline immediately. Tetracycline treatment can last up to ten days. Some experts argue that all foods should be immediately changed and that any remaining existing foods be disposed of. You may want to elevate your aquarium temperature to 80°F during that time if you do not have any fish that are intolerant of high temperatures. Furunculosis is a cold-water disease and the high temperature is thought to kill it.

  • Ulcers (Hole-in-the-Body Disease; highly contagious)

    This is an infection that tends to be internal and that manifests itself in large red ulcers, boils, and dark reddening at the bases of the fins. It cannot be mistaken for anchor worms, which are covered later, because anchor-worm ulcers swell up, whereas these tend to eat away into the body.

    A salt bath may be too harsh, but the infected fish should be isolated immediately and fed medicated food. At times, antibiotics are required, and you will need a veterinarian for this. Consult your local pet store before proceeding.

  • Fungus (highly contagious)

    The most common species of fungus infecting tropical fishes is Saprolegnia. It is a fuzzy growth, different from velvet (see below) because it is whiter and easier to notice. The primary cause of this infection is damage to the mucus layer on the skin. This allows fungal spores to germinate and grow into the skin. Injury, environmental conditions, and parasites can damage the protective mucus layer.

    Some experts treat this fungus with methylene blue, which they paint on the infected areas. This fish is then placed in a 10-day saltwater treatment. Again, commercial remedies are also available and the entire aquarium should be treated with a fungicide.

  • Body Slime Fungus (highly contagious)

    This deadly affliction can kill your fish in two days if not caught in time. The protective mucus coating grows white and starts peeling off as if the fish were shedding its skin. The fins are gradually covered as well. Eventually, the body becomes red with irritation.

    Do not hesitate to call your pet store immediately. Commercial remedies are available, but must be administered quickly. A salt bath with warm temperatures may be a temporary solution, as it should retard growth of the fungus. However, a cure must be found and a salt bath won't do it. Some have found that the severe salt treatments used as ich cures are effective.

  • Mouth Fungus (Columnaris Disease; contagious)

    Mouth fungus is caused by the bacteria Flexibacter and manifests itself as a white cottony growth on the mouth. It can also be associated with the gills, back, and fins. If left untreated for any length of time, this infection will destroy the entire infected region and lead to death.

    Commercial cures are available, but you can begin by isolating the fish and administering the saltwater treatment. Some aquarists will start with a salt bath, then use a general fungal or bacterial control. Consult with your pet store once you have diagnosed the problem.

  • Fish Pox (probably not contagious)

    Fish pox affects goldfish, koi, and carp more than it does other aquarium fish, but it should be covered in this chapter. This is a viral infection that causes milky white or pinkish gray waxy film to develop over the fish's skin and fins. The usual pattern is that it appears, gets worse, and then disappears.

    It is not definitively known what triggers fish pox and what eventually happens. However, it does not appear to be contagious. Nonetheless, take the necessary precautions and isolate the infected fish until the film goes away. This usually takes seven to ten days. This ailment is more annoying then anything else since it does not kill the fish. However, there is no known cure.

  • Fin or Tail Rot (contagious)

    This is sometimes caused by fighting among your fish—the fins get damaged and then bacteria infect the injured area. It can also be triggered by poor water quality. It is easily detectable as the fins have missing parts and eventually become shredded. As the disease worsens, the entire fin will be eaten away. There are many broad-spectrum medications that will help you deal with this situation. Consult your local pet store dealer.

    Some experts argue that the best way to treat the infection is by dipping your fish into a bath made up of eight crystals of potassium permanganate to three quarts of water for five minutes. Then you cut off the infected areas of the tail or fin and paint them with methylene blue or mercurochrome. Steps like this may be for experts only.

    Be sure to treat the aquarium water, too, because fin rot is usually contagious. You can choose from any number of commercial solutions available in pet stores today. Follow the instructions. Also, take the steps necessary to remedy the cause of the infection. Separate fish that cause injury to the fins and make sure water quality is at its best.

  • China Disease (highly contagious)

    This is not a very common disease, and you must be absolutely certain of your diagnosis. This is the most contagious disease listed here and it is the most deadly. There is no known cure for China disease.

    The symptoms are very easy to spot. The tail fin and other fins begin to fray, very much as in fin rot. However, with China disease it begins at the base of the fin and works its way outward. Also, the infected areas begin to blacken. Even the ventral region begins to turn black.

    Unfortunately, the infected fish must be painlessly destroyed and the other fish put in the hospital tank. A 10-day progressive salt treatment isn't a bad idea.

    In the meantime, you need to perform an emergency cleaning in the tank. This must be done immediately to prevent further damage by this disease.

Parasite Infestations

  • Fish Lice (highly contagious)

    Fish lice are parasitic crustaceans of the species Argulus that are very easy to recognize on the surface of your fish. They are round disk-like crustaceans with prominent eyes, sucking disks, and a stiletto mouthpart that clamps onto its host, refusing to let go. They can move about the host with ease and tend to take on the color of the fish that they parasitize.

    Often the infected fish will rub up against objects in the tank in an effort to scrape these pests off. Some fish have been known to jump out of the water in an attempt to cleanse themselves of these parasites. These creatures feed by sucking the blood and tissue fluids out of the fish through the skin and scales. Sometimes they occur on the fins, but this is not as satisfying for them. Fish lice can also transmit other microscopic diseases, and wounds may develop secondary bacterial infections.

    Fortunately, there are a number of high-quality commercially produced products on the market to control parasites. Your local pet dealer can help you select one. The fish should be quarantined and the tank disinfected with the same parasite control. Make sure to follow the directions carefully, as overdoing the treatment is just as damaging.

    On larger fish, experts have been known to remove fish lice with forceps or drip hot paraffin wax from a candle onto the parasite. Others recommend giving the afflicted fish a bath for fifteen minutes in a mixture of potassium permanganate and water, which should be extremely light pink. Consult your local pet store first. Lice are treatable, but both the fish and the aquarium must be treated.

    Most often recommended for aquarium treatment for fish lice, anchor worms, and leeches are Dipterex, Masoten, Dylox, or Nequvon. All bite marks or wounds on the fish must be treated. Dab a little antiseptic m0ercurochrome, malachite green, or methylene blue on the site. Do not use Formalin to kill the parasite; you may also kill your fish because the margin for error is so slim.

  • Final Rites-Saying Goodbye

    What do I do with dead fish? Well, there have been various things that many pet owners have done.

    Some folks flush away their sadness (along with their loved ones). Do this with small fish only.

    Some people bury them in the backyard. If you do this, there's a 90% chance that some roving canine will express his interest in your loved one.

  • Anchor Worm (highly contagious)

    These elongated crustaceans of the genus Lernaea also attach to the skin of the fish. Several species of this parasite have been described, but all females have a head with an anchor shape that embeds in the flesh of the host. The fish will rub against anything it can in an attempt to scrape off the parasite. Like fish lice, these creatures cause irritation and localized bleeding at the point of attachment; from this protrudes a white worm that can sometimes grow quite long. Secondary bacterial infection can occur at these points.

    Treatment of the anchor worm will include taking the fish out of the water and removing the worm from the aggravated area with a forceps or tweezers. Be sure to carefully follow the instructions accompanying any parasite-control product that you buy.

    To remove the worm, place a wet cloth in your hand. Hold the fish in the hand holding the cloth. Make sure to position the fish so that the worm is facing you. With a pair of household tweezers, gently press as close to the ulcer as possible, but only extract the worm. Be sure not to rip any flesh off the fish and be careful not to break the parasite. This is very dangerous to the fish and you must be extremely cautious when approaching this. It is best to get someone experienced to do this for you.

    As in the case with fish lice, be sure to treat the infected area with an antiseptic after removing the parasite. Antibiotic treatment may also accelerate the healing of lesions. Consult your dealer for a general full-spectrum antibiotic.

  • Leeches (highly contagious)

    Leeches are another group of parasites that may be found on the skin and scales of your fish. These are not the leeches we see as free living creatures in lakes and ponds. These are parasitic, wormlike creatures that attach at both ends to your fish, feeding on flesh and blood. They need to be removed as quickly as possible, but not with forceps or tweezers. These parasites are very strong, and you are likely to do more damage to your fish than to the leeches by trying to pull them off. Call your pet store for advice for commercially produced cures.

    Another solution to leeches involves preparing a salt bath with eight level teaspoons of salt for each gallon of water. Once the salt is sufficiently dissolved, add the fish for no more than ten minutes. The leeches that do not fall off can now be removed with tweezers very easily.

    Again, the aquarium needs to be treated immediately with commercially produced chemicals for parasite control. Check all your fish for parasites when one is discovered and always isolate the infected fish.

  • Flukes: Skin and Gill (highly contagious)

    Like all infestations, weakened fish fall victim first. The gill fluke (Dactylogyrus) is easily detectable. It causes the gills to swell up pink and red, and the fish spends a lot of time near the surface trying to suck in air. Sometimes, a pus-like fluid will be exuded from the gills at this time. These flukes are microscopic parasites that lodge themselves in the gills. Other symptoms include severe color loss, scratching, and labored respiration. The skin fluke (Gyrodactylus) causes localized swelling, excessive mucus, and ulcerations.

    As with all other parasitic manifestations, the host fish is constantly trying to rub itself against objects to scrape off the infestation. Again, pet stores have pest-control remedies for this problem, which is more easily treatable than the ones I have already listed. Be sure to treat the tank as well to make sure that this parasite does not spread.

    Some experts recommend a formaldehyde bath. Do this only if commercial solutions are unavailable or are not effective. Place the fish in a gallon of water. Add 15 drops of formaldehyde every minute for 10 minutes. Then remove the fish and place it in a hospital tank. Repeat this process daily for three days. Formaldehyde will kill your fish, so do not haphazardly administer this chemical. Follow the instructions and time it precisely.

  • Ich (White Spot; highly contagious)

    Raised white spots about the size of salt or granules that appear on the body are the parasite Ichthyophthirius. This is one of the most common parasites among aquarium fish. It should not be taken lightly, as it will kill your fish if given enough time.

    This ailment is so common that there are many commercial ich remedies on the market. Many of them are good, so don't buy the cheapest one, buy the best. Follow standard procedures and remove the fish showing the symptoms and treat it in a hospital tank. However, the entire aquarium must be treated as well. Follow the directions carefully.

    If an ich treatment is not available to you, raise the aquarium temperature to 85°F and add one teaspoon of salt for every gallon of water in the tank. Give the infected fish in the hospital tank the 10-day saltwater treatment. It is important to kill this organism before it has an opportunity to infest the entire population.

  • Velvet (highly contagious)

    The parasite Oodinium causes a golden velvety coat on the body and fins, which is referred to as velvet. In orange-colored fish, like goldfish, velvet is sometimes very difficult to detect at first. Commercially produced remedies are best for this parasitic affliction. Some experts use malachite green or the old-fashioned 10-day salt bath. Use the commercial product, but if one is not available, try the salt bath. You should administer some kind of antifungal chemical in the water of the aquarium to disinfect the tank as well.

  • Hole-in-the-Head Disease (contagious)

    This disease is caused by the parasite Hexamita, which is an internal parasite that is harmful when the fish is weakened by stress, age, or poor water quality. It is generally characterized by white stringy feces and enlarged pus-filled sensory pores in the head. Other symptoms include erosion of the skin and muscles that eventually extends to the bones and skull. The lateral line is also a preferred site for these lesions.

    Transferring the fish to larger tanks and implementing frequent water changes is sometimes enough to cure the fish. Improved nutrition supplemented with vitamin C has been known to improve the condition as well. The prescription drug metronidazole prepared in a bath of 50 mg for every gallon of water is effective in treating this disease. It is recommended that you repeat this treatment after three days.


If a red agitated area on the fish's body is the base from which a white worm protrudes, and the diseased fish rubs against anything it can, attempting to scratch off the parasite, then your fish has ANCHOR WORM.

If the protective skin mucus grows white and starts peeling off, as if the fish was shedding or molting, and the fins are eventually covered as well, then your fish has BODY SLIME FUNGUS.

If, at the beginning at the base of the fins and working its way outward, tail fins and other fins begin to fray, infected areas begin to blacken, and the ventral region begins to turn black, your fish has CHINA DISEASE.

If your fish is very inactive, usually rests on bottom of the tank, and abdominal swelling and bulging occurs, then your fish has CONSTIPATION, INDIGESTION. If the scales stick out like pinecones and the abdomen bloats noticeably, your fish has DROPSY (KIDNEY BLOAT).

If, at first, the fin have missing parts, then they become shredded, and rays become inflamed and the entire fin may be eaten away, then your fish has FIN OR TAIL ROT.

If round, disk-shaped transparent crustaceans clamp onto the host fish and refuse to let go, and the infected fish rubs against objects in the tank in an effort to remove the parasites, your fish has FISH LICE.

If whitish or pinkish waxy film develops over fish's skin and fins, then your fish has FISH POX.

If your fish has a fuzzy growth, different from velvet because it is more whitish in coloration, then your fish has FUNGUS.

If your fish has raised bumps under the scales that eventually rupture and cause bleeding ulcers, then your fish has FURUNCULOSIS.

If your fish's gills swell pink and red, and the fish spends time at the surface gasping for air, and pus-like fluid is exuded from the gills, then your fish has GILL FLUKES.

If your fish has white stringy feces and enlarged pus-filled sensory pores in the head, plus erosion of the skin and muscles, which eventually extends to the bones and skull, then your fish has HOLE-IN-THE-HEAD.

If your fish exhibits raised white spots about the size of a salt granule on the body and fins, then your fish has ICH.

If your fish has long worm-like parasites attached at both ends to the fish, which do not come off easily, then your fish has LEECHES.

If white cottony growth on the mouth sometimes spreads to the gills and other parts, then your fish has MOUTH FUNGUS.

If the fish's eyes protrude from an inflamed eye socket, then your fish has POP-EYE.

If your fish exhibits ulcerations on the skin, localized swelled areas, and excessive mucus, and if the fish is constantly trying to rid itself of these parasites by rubbing against aquarium objects, your fish has SKIN FLUKE.

Does your fish swim on its sides, upside-down, or turn somersaults in the water? And can it be found either on the top or the bottom of the tank? Then your fish has SWIM BLADDER DISEASE.

Does your fish have obvious bumps, lumps, or protrusions that sometimes look like a large blister of wart? Then your fish has TUMORS.

Does your fish have large red lesions, boils, dark reddening, and bleeding? Then your fish has ULCERS.

Does your fish have fuzzy areas that grow with a yellow or golden color? Then your fish has VELVET.

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