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Common Fish Diseases and Conditions

Eileen here. Please enjoy the 4th installment of my fish glossary of terms (Check out part 1, 2, and 3). We will be publishing it in its entirety at some point.

Common Diseases and Conditions:

Ammonia Burn: As the name suggests, this is a burn-like injury caused by highly acidic ammonia build-up. Ammonia can cause open wounds on a fish’s body or damage to sensitive structures like their gills. These excessive ammonia levels are commonly caused by poor filtration or overstocking with too many fish or fish too large for the tank. Treatment includes eliminating the cause of the high ammonia level, neutralizing or removing the ammonia, and treating the wounds if necessary to avoid infection.

Aeromonas: Aeromonas infections are caused by several species of bacteria that are opportunistic and will affect organisms with weakened immune systems. It can affect fish, amphibians and even humans in some cases. Koi and other pond fish are vulnerable to aeromonas infections during the early spring and summer when temperature fluctuations can leave them vulnerable and weakened. Aeromonas attack organs and will digest gelatin and hemoglobin cells. It often appears as deep open sores on the body of a fish as well as causing severe weight loss as it attacks the internal organs. Aeromonas bacteria is very resistent to most medications and can be very difficult to treat. Strong gram-negative bacterial medications both in the water and in food treatments can be used. Injections are also sometimes used by veterinarians and biologists to treat larger fish.

Bacterial Infection: Bacterial infections can have a number of causes and symptoms. They can result from poor water conditions or an injury, or as a secondary infection from a parasite or fungal infection. Some infections can be diagnosed as “gram-positive” or “gram-negative” in specific cases, but most are simply general infections. The most common symptom of a bacterial infection is a reddish patch or sore on the body of the fish or as reddish streaks in its fins. A medication is usually needed as treatment and can range from mild botanical-based solutions to stronger antibiotics including sulfa, penicillin and amoxycillin. A bacterial infection is almost always a symptom of a larger problem (bullying within tankmates, poor water conditions, parasites, etc.).

“Black Ich” or “Black Spot”: This disease is a parasite infection caused by flatworms. It mainly affects tangs and surgeonfish and appears as small dark spots on the body of the tang. The fish may also flick or scratch against surfaces or may be less active than normal. The flatworm lays eggs on the body of the fish and drops off within a few days, leaving the eggs to hatch on the fish a few days after that. Treatment for Black Ich can include freshwater baths or antiparasitic medications with active ingredients like formalin.

Brooklynella: This parasitic infection is also known as “Anemonefish Disease” or “Clownfish Disease” due to its most common victim. It is a protozoan that usually spreads very quickly and is almost always fatal and has no commercially-affective treatments. It can first be seen as a fine sheen on the affected fish – usually newly captured or transported fish – but soom evolves to signs of physical stress to the fish, difficulty breathing, and excessive slime coat production. As this slime coat sloughs off the fish, it can spread the protozoans throughout a system to prompt quarantine of an infected fish is absolutely important. All tank equipment should be cleaned and sterilized well as well to avoid spreading the disease. As I mentioned, there are no medications that are known to be very effective on these protozoans, but medications like those containing formalehyde, malachite green and and methylene blue used in a quarantine tank can help. Do not use freshwater dips with this infection.

Environmental Stress: Environmental stress can result from improper water conditions (temperature, pH, salinity, etc.), unsuitable décor (too much or too little vegetation or decorations, too intense or dim lighting, etc.), or exterior conditions like activity in the room around the tank or vibrations caused by an unstable surface or tapping on the tank. Some of these stresses can be easiliy fixed while others can be more difficult. Poor health or unusual behavior (including jumping from or trying to jump from the tank) can hint towards something wrong with the fish’s environment. Researching all choices for an aquarium and observing behavior regularly can help an aquarist to notice these problems.

Fin Rot: Fin rot is more of a symptom than a disease and is essentially just what the name describes. The fins on the fish, most often first noticed in the caudal (tail) fin, will appear to be rotting away and may be red and ragged. The fins may also appear white as a secondary fungal infection sets in in some cases. This is usually caused by a bacterial infection and can be the result of bullying or fin-nipping tankmates or poor water conditions. The cause of the condition should be address (water quality improved or aggressive tankmate removed) and the fish can be treated with an antibiotic or antibacterial medication.

Fungus: A fungal infection in aquarium fish is more often a secondary result of another condition rather than a separate condition in itself. Secondary fungal infections usually result when a fish is already weakened due to poor water conditions or another disease or condition. Leftover food or waste in an aquarium can also cause fungal growth. Fungus in aquarium usually looks like white cottony tufts on the fish or on the waste and can be treated with antifungal medications and by eliminating the cause of the fungus (poor water conditions, overfeeding, wounds due to parasites or aggression, etc.).

Head and Lateral Line Erosion (HLLE): Head and Lateral Line Erosion (HLLE) is seen mostly in saltwater tangs and angelfish but other fish are also susceptible. Another similar condition, known as Hole-in-the-head, is very similar in appearance but mainly affects large cichlids and freshwater fish. With HLLE, the area around the head and eyes and the length of the lateral line down each side of the fish’s body becomes pitted and can appear to be rotting away. While not usually fatal in itself, HLLE can cause permenant scarring and is a symptom of a more serious and ongoing condition like poor nutrition. Fish like tangs that are especially vulnerable to this condition should be fed a varied diet high in fresh macroalgae. Vitamin supplements can also be helpful. Some research suggests that HLLE can also be caused at least in part by other stresses like some flagellate parasites or stray electrical current.

Hole-in-the-head Disease: Hole-in-the-head is very similar to HLLE, described above. Some aquarists argue that these two names refer to the freshwater and saltwater versions of the same condition since they share some of the same causes and symptoms. Hole-in-the-head is found mostly with large cichlids like Oscars and Discus but can affect other fish as well. Like HLLE, the main cause is usually linked to improper diet or water conditions and adding vitamin supplements and a varied diet can often help stop and reverse some of the effects of the disease. Hole-in-the-head is also attributed more to parasite infections than HLLE, specifically protozoans.

“Ich”: Ichthyophthirius multifiliis, also known as Ich, Ick, or White Spot, is one of the most common and well-known conditions in the aquarium hobby. Many aquarists incorrectly diagnose problems with their aquarium as ich when it is actually another condition. Ich is a parasite that affects freshwater fish (as opposed to the similar Cryptocaryon that affects saltwater fish but has an almost identical appearance, symptoms and treatment). Ich appears as small white spots on the body of the fish. These spots look like grains of salt as opposed to the cottony tufts of fungal infections, the dust-like appearance of oodinium or some bacterial infections or the pits associated with Hole-in-the-head and HLLE. The fish may stop eating, may scratch and rub against surfaces, and may appear lethargic. Treatments include increasing temperature, adding vitamin and garlic supplements, freshwater or saltwater “dips”, and a score of medications. The parasite also has a life cycle that allows it to remain dormant in an aquarium or on a fish for weeks at a time before blooming when triggered by environmental conditions or the fish’s immune system being weakened by stress or anther condition. Some medications available to treat ich are not safe for all fish or for invertebrates; be sure to choose the medication suitable for your aquarium.

“Lymph”: Lymphocystis, commonly known as “lymph”, is a virus that affects both freshwater and saltwater fish. This virus forms white cauliflower-like growths on the fins and body of the fish and can cause white patches around the eyes. It usually develops when the immune system of the fish is weakened due to poor nutrition or water quality. Being a virus, there is no reliable medication to treat it but improving the conditions and diet will help to boost the fish’s natural immune system and can help the fish fight it on their own. Some sources recommend removing the nodules by scraping them from the body or fins; this can be a dangerous approach that may lead to excessive stress and secondary infections. Lymph is not usually fatal and often may clear up on its own as the fish fights off the virus.

Marine Velvet/ Oodinium: Marine Velvet, also known as Oodinium, is caused by the parasite Amyloodinium ocellatum. This is one of the most fatal parasitic infections as it is very contagious and resistant to most medications. Marine Velvet looks like a very fine velvety coating on the fish as opposed to the salt-like spotting of Ich. The fish may also breathe rapidly and have cloudy eyes. Quarantine and treat any affected fish as soon as possible with a strong antiparasitic treatment like copper sulfates. The salinity can also be lowered and a UV sterilizer can be used to help kill the parasites.

Parasites: Parasites in general are one of the most common aquarium maladies next to unsuitable water conditions. Parasites by definition are any organism that requires a host organism to live, often to the detriment of that host and may lead to the death of the host. The size of parasites varies greatly from tiny organisms like those responsible for Ich and Marine Velvet, to larger crustaceans and worms like flukes, anchor worms or fish lice. These parasites can also be external (living on the outside of the body of the fish) or internal (living inside the fish, often in organs like the digestive system). Common signs of parasites are rapid breathing, weight loss, white feces, sores or a flicking or scratching behavior against rocks and surfaces. Different parasites require different treatments and treatment methods and so should be carefully diagnosed before medicating or treatments.

Pop-eye: Like the name suggests, “pop-eye” is a condition in which one or both eyes of the fish appear to be swollen and popping out of the socket. This can result from a bacterial infection, poor water quality, injury or in rare cases a gas pocket in the eye socket. This is typically a symptom of an underlying condition like poor water quality and is difficult to treat specifically. Anti-bacterial medications may help but the water quality and any other possible causes should be addresses as well. Pop-eye may often clear on its own but may lead to decreased vision in the affected eye.

Swim Bladder Infection/Disorders: This condition is seen mostly in fancy goldfish, balloon mollies and other fish bred for a similar “chubby” body shape. The swim bladder in fish is used to help control buoyancy (the up and down motion) through gas exchange within the fish’s body. The fish is considered “neutrally buoyant” when it is able to hover in the water without floating up or sinking down. If the fish is unable to achieve buoyancy, it may float, sink or be unable to swim properly or remain upright in the water. This may be the result of an internal bacterial infection that can be treated with some medications. Some fish, especially fancy goldfish, may gulp air at the surface will feeding and have similar issues caused by air in their digestive systems. Feeding them fresh greens like peas or zucchini may help get rid of this bubble.


  1. avatar

    I have a coi. About 2.5″. He was taken from a very toxic tank and resides alone in a 10 gallon tank. He is showing signs of fin rot. Tail and top fin. There are no signs of bleeding or infection. Testing of his tank shows a clean environment. What should be my next step to help him with his issue. What do I need to be aware of as he heals. Will the fins grow back to health and approximately how long does that process take?
    Is a weekly 25% water change enough given he has no serious signs of illness?

    Also, I lost five fish last week in my 55 gallon tank. sadly, I was not at all prepared for them with knowledge. Ammonia got completely out of control and it was too late to save them. Now I am doing weekly testing, frequent water changes, sweeping the bottom,… I have three sucker fish who continue to live in there, showing no signs of stress or illness. I did do an ich treatment last week as well. What do I need to be doing and how often to get this tank ready for some beautiful fish, to live happy and healthy in this beautiful tank? Also, we are rounding the final week or two of cycling. When I change the filter, do I want to just remove the ammo carb from the filter, and keep the filter so I don’t throw out all the beneficial bacteria in the tank?

    Thank you for your help.

  2. avatar

    Hello Tracy, As far as the fin rot, you would want to get a medication to treat it and follow the directions on the medication. An Anti-Bacterial Medication would be your best bet for that. The medication you choose will have directions on when to do water changes and how long to medicate and at what dosage. For the other tank, I wouldn’t recommend medicating unless you see signs of the condition you are medicating for; if you don’t see any signs of ich for example, you wouldn’t need to medicate for it. Bacterial filter material would not need to be changed; only chemical filtration like your Ammo-Carb or carbon would need to be changed regularly and mechanical filtration like filter pads would need to be changed or rinsed. You can find a lot of General Aquarium Care tips on our website’s Article Archive that will help you maintain your tank, and you can call and speak with someone in our fish room at 717-299-5691 if you would like some more specific advice on your aquarium maintenance and stocking.

  3. avatar

    Thank you Eileen.

    In my 55 gallon tank where 3 sucker fish continue to reside, high ammonia persists. Reading between 1.0 & 2.0 ppm. Today I did a 50% water change. I also emptied the filters of old ammo carb and added the recommended amount for my size tank of new ammo carb.
    How soon should I test the water again, and how long is this going to take to resolve? I am getting frustrated. I purchased this tank (brand new) and filled it the last week of December. I have done many water changes, and although the ammonia has dissipated a bit,(was 4.0 or higher when I lost five fish) it is still too high for me to even think about putting any fish in there. I don’t even know how the sucker fish are surviving. But they are watched closely and seem to be doing fine.
    However, then I wonder what do I feed them? There are obviously no fish to feed. I do have algae wafers for them, but they don’t seem to even touch them…they just blow up like clouds…if that is sufficient, how much should I give them of the wafers per day?

    Finally, Do you sell the medication for fin rot at the store?

    Thank you for all your help.

  4. avatar

    Hello Tracy, We sell many medications that treat Fin Rot. The link I provided in my previous reply to Anti-Bacterial Medications has links to all of the different medications we offer. Your tank will have to go through the cycling process again since you’ve essentially restarted it with doing large water changes. You can add a supplement like Nite-Out to help this process along but it will take time and doing water changes during this time will extend it. I would recommend contacting someone in our Fish Room at 717-299-5691 or 888-842-8738 so that they can discuss this with you in more detail than I can with a reply here.

  5. avatar

    I called the store. Thank you so very much for your help.

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About Eileen Daub

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Marine Biologist/Aquatic Husbandry Manager I was one of those kids who said "I want to be a marine biologist when I grow up!"....except then I actually became one. After a brief time at the United States Coast Guard Academy, I graduated from Coastal Carolina University in Myrtle Beach, South Carolina in 2004. Since then, I've been a marine biologist at That Fish Place - That Pet Place, along with a Fish Room supervisor, copywriter, livestock inventory controller, livestock mail-order supervisor and other duties here and there. I also spent eight seasons as a professional actress with the Pennsylvania Renaissance Faire and in other local roles. If that isn't bad enough, I'm a proud Crazy Hockey Fan (go Flyers and go Hershey Bears!).