Why Are My Fish Dying? A Guide to Diagnosing Fish Diseases.

Updated: Nov 20, 2021



This is not an article on how to treat fish diseases, but rather a guide to the process of diagnosing common fish diseases. Too often I notice people online confusing bacterial infections with protozoan parasites, mistaking the sloughing of slime coating for fungus. Being able to properly diagnose and recognize which infection, disease, or sickness your fish has will greatly increase your chances of successfully treating it. Included at the end of this article is a breakdown of the symptoms for a few common fish diseases.


The key to diagnosing fish disease is accumulating as much data as possible, and avoiding jumping to conclusions and making false causations. Accumulating data involves watching your fish, noticing patterns, and taking notes of every symptom you notice. Avoiding jumping to conclusions means waiting until you have as much data as possible before making a diagnosis. Don't assume anything based on one symptom or behavior. Avoiding false causations refers to mistaking correlation for causation. Just because two events happened simultaneously doesn't mean they happened because of the other.


For example, lets say you had an ich infection recently and a few fish died. You cleared up the infection and the remaining fish seem to be healthy. A few days later, you notice a couple fish hovering at the surface of the water and breathing heavily. You might assume the ich parasite is back and your fish are infected again. However, after a quick ammonia test, you find out that one of the expired fish was not removed and decayed, releasing ammonia into the tank. These elevated ammonia levels are what caused the heavy breathing and hanging out at the surface. If you had jumped to the false conclusion of ich returning, you would have treated the tank for ich again instead of doing an immediate water change to alleviate the ammonia levels.


False causations can be made quite easily, the key is to always consider another possibility other than the obvious one, and then test for it. You'd be surprised how much headache you'll save yourself.


You're the Doctor

Let's say you have a sore throat and your nose is running a little bit. Is it just allergies or something more serious? It doesn't clear up in a day or two so you make an appointment with the doctor. During your visit, the doctor may check your breathing and ask several questions like; how long have you had the sore throat? Are you coughing? Do you have a fever? Is there mucus accumulation in your lungs? Are you congested? The doc is trying to ascertain what the possible cause of your discomfort is based on the symptoms your experiencing. She can rule out certain infections, bacterial strains, or viruses based on what symptom you are, and are not showing.


This is the exact same process we must go through with our fish. We have to make a list of symptoms to compare them to the known symptoms of certain diseases. Showing white spots and clamped fins? That is probably ich and not a viral infection. Cloudy eyes and sloughing of the slime coating? That is probably a bacterial infection and not fungal.


Our job can be harder than the doctor's though. Unlike your doctor's office, we do not have a lab to test for viral or bacterial infections. Your doctor may say, well, its either strep throat or a viral infection. Then she sends some saliva to the lab and they rule out strep real quick. We do not have that luxury. That is why it is very important to pay close attention to the symptoms being exhibited by our inflicted fish. It could be the difference between life and death. Wasting time mistreating a disease they don't have is a luxury they can't afford.


Most symptoms fish exhibit when sick are shown with multiple infections. Lethargy and clamped fins, for example, are common with most if not all diseases. Trying to diagnose your fish based of one or two symptoms is much less accurate than identifying as many symptoms as possible. The most accurate diagnoses will include at least 50% of the symptoms listed for each infection. (See the list of symptoms for each disease below).


Once you have your list of symptoms, it's time to carefully cross-reference them with possible diseases. If you don't have enough data to narrow it down to a specific bacteria, fungus, or parasite, know that all three of these are biotic infections and can be treated with a general antibiotic. If you don't know what type of fungal infection is in your tank, but you know it to be fungal in nature, hydrogen peroxide and an anti-fungal medication are a good start.


My point is, unless it is a common or easily identifiable infection like ich, you may not know exactly what the cause is. In these cases, you should be able to at least identify the classification or kingdom the infection belongs to; bacterial, fungal, protozoan, or viral. Armed with this knowledge, your treatment plan can start from a place of understanding and knowledge instead of the "throw it and see what sticks" approach.


Accumulating data and avoiding making assumptions are all you need to properly diagnose your common fish diseases. Below I've listed some of the most common infections and their known symptoms.


Ichthyophthirius multifiliis (ich parasite) - parasitic

  • anorexia (loss of appetite)

  • hyperventilation (heavy breathing)

  • discoloration

  • lethargy (inactivity)

  • flashing (quickly scraping or rubbing flank against gravel or decor)

  • unbalanced ( lilting or swimming upside down)

  • white spots (salt sized white specs sprinkled over body)

Columnaris (cottonmouth) - bacterial

  • frayed or ragged fins.

  • skin Ulcers (open wounds)

  • white or cloudy, fungus-like patches on gill filaments and ulcers.

  • accumulation of mucus on gills, head, and dorsal area.

  • gill filaments turn brown (instead of red), with tissue death.

  • hyperventilation

  • anorexia

  • lethargy

Dactylogyrus (gill or skin flukes) - parasitic

  • hyperventilation

  • clamped fins

  • flashing

  • noticeable fluke parasites on gill filaments.

  • accumulation of mucus coating

Dropsy - bacterial, environmental

  • swollen abdomen

  • furled scales (scales stick out from body)

  • bulging eyes

  • discolored/pale gill filaments

  • swollen, red anus

  • pale feces

  • skin ulcers

  • lethargy

  • anorexia

Piscinoodinium (velvet) - parasitic

  • lethargy

  • hyperventilation

  • anorexia

  • clamped fins

  • yellow-white, gold-brown colored dust covering the body

  • skin peeling / ulcers

  • flashing

Hexamita (hole in the head) - parasitic

  • indentions/dents in the top of the head which can become severe lesions several mm deep.

  • white, stringy feces

  • pale coloration

  • anorexia

Some conditions not listed, like pop eye, swim bladder disease, and fin rot, are actually more commonly symptoms of diseases or poor water conditions, rather than from a specific pathogen.

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