Diseases In Fish: How Do I Treat Them?

It is extremely important to detect disease at the earliest possible opportunity, since prompt treatment may be able to save the affected individual fish and equally important, prevent the disease spreading to other fish.

Many people ask me how you can tell when a fish is sick. My response is: ‘look with your eyes’. The ability to recognise signs of illness relies upon regular observation of your fish – to be familiar with their normal physical appearance and normal behaviour. Recognising abnormality is only the first step. Since most clinical signs of ill health are non-specific, further work is required to reach a diagnosis and create a treatment plan.

In fresh water, the internal tissues of fish are hyper-osmotic to their environment, whereas in salt water they are hypo-osmotic. What this basically means is that if you get a break in the skin of a freshwater fish, it will act like a sponge and swell up with water. And if it is a marine fish, the opposite will happen, it will dehydrate. Skin injuries make osmoregulation more difficult for the fish and can become critical due to loss of electrolyte and fluid balance, ultimately leading to circulatory collapse.

The gill has three primary functions – respiration, excretion (monovalent ions and nitrogenous excretions) and fluid balance. The very large surface area afforded by gills allows these functions to occur efficiently. However, the delicate structure and exposure of the gills make them very prone to damage and given its many key physiological functions the consequences can become quickly critical. Thus, it is very important to seek professional advice immediately. Delaying treatment or giving the wrong treatment creates a life or death situation.

Any fish in a tank that gets sick, regardless of its monetary value, needs medical attention immediately. This is not only a welfare issue, but the sick animal acts as an indicator to the health of your entire aquarium and all the fish in it. Any delays or improper medication often exacerbates the condition and predisposes the rest of the fish’s tank mates to stress and risk of losing them to disease.

I am a strong believer that prevention is better than cure – and this philosophy is particularly pertinent to aquatic medicine as the range of treatments is very limited compared to that available for terrestrial animals.

To illustrate the non-specific nature of the clinical signs of fish diseases, a case study is presented below.

Case history

A new tank had been set up for about three weeks. A little goldfish had profuse hyperaemia (redness) around the ventrum and gill covers, was lethargic and appeared depressed. The fish was not eating and was much less active than normal. There were no gross signs of excessive mucus production on skin and respiration appeared normal. The fish was not ‘flashing’.

Possible causes of disease

Dirty water, parasites (fluke or protozoa), bacterial infection, fungal infection, heavy metal toxicity… which of these would you pick as the problem? Can you be sure? Each one of these requires a different course of treatment. How would you go about treating this fish?

The correct answer was only reached through a thorough investigation – that case was bacterial septicaemia, and I treated the fish successfully with some prescription antibiotics. But 2 weeks later… another fish in the same tank fell sick.

Clinical signs

The second fish was not eating because it couldn’t open its mouth and it looked like it couldn’t see because it had some sort of tufty growth over the eyes. The owners isolated the fish and put it in some aquarium salts and tried to use the leftover antibiotics on the fish, but unfortunately it died.

A few things can be learnt from this exercise:

  • The right diagnosis means you need specialised equipment and experience;
  • Different diseases require different treatment regimes;
  • If you get the diagnosis wrong, and use the wrong medication, you will end up killing your fish – wasting time and money.

Quite a few fish books and internet sites have a ‘DIY diagnosis chart’. While I think that it is a good way to start an aquarist thinking, there is no sure way of identifying the disease without having the proper equipment and experience. Some have simplified the process to a dichotomous key. One wrong step in the process can give you no result, or worse still, give you erroneous diagnoses causing you to do more harm than good for your fish. Also, a word of warning about internet sites. Most sites are full of ‘information’, however, some information may not be entirely accurate and may be misleading. Any Tom, Dick or Harry can create sites on the internet. How can you be so sure that the information you have gathered is reliable?

Many diseases come about as a result of several factors such as environment, nutrition and care. To solve these sorts of problems requires a multi-disciplinary approach. Veterinarians have the right training for making an accurate diagnosis and prescribing a specific treatment for each problem. Veterinarians possess the relevant background in microbiology, parasitology, anaesthesia, surgery, physiology, anatomy, pathology, pharmacology and toxicology to carry out a wide variety of diagnostic services for a pet owner.

I would like to emphasise the point that clinical signs of disease in fish are often non-specific. Two fish that present with the same clinical signs may be affected by two completely different diseases. Another fish I had seen that was presenting with the exact same clinical signs was affected not by bacteria but a flatworm infestation. So, the clinical signs may be similar but the causative agent and hence treatment options will vary.

There is no time to waste. Treat them right the first time otherwise they will die. Most medications act on the basis that they are more toxic to the bug than they are to the fish (i.e. killing the bug before it kills the fish) and the margin of safety can be quite low.

From personal experience, I have found that more pet fish die as a result of aquarists administering wrong treatments. There are a number of reasons why this is so:

  1. Wrong drug – bugs live, but fish die.
  2. Right drug, too low a dose – bugs live to kill the fish.
  3. Right drug, too high dose – kill bugs, but also kill fish (more is not better).

There are many side-effects and toxicity problems and only a person trained in pharmacology and toxicology can tell what they are.

Which brings me to my next point…


Drugs used to treat fish can be divided into 2 categories: non-selective and selective.

Non-selective – kills virtually anything that is living. These treatments work on the idea that the concentrations needed to kill the bugs are slightly less than what would kill a fish. Since it does not differentiate between fish and bugs, it is VERY IMPORTANT TO FOLLOW THE DOSE RATES prescribed on the label. NEVER OVER-DOSE. More is NOT better, e.g. formalin.

Selective drugs can be much safer to use, however, dose rates should still be followed because, although they are selectively toxic to the bug, they may have toxic side effects often relating to liver or kidney function. The database of long-term toxicity studies in fish is very limited so caution is urged.

Another issue regarding the responsible use of drugs is the development of drug resistance. I’m sure you’ve all heard of  ‘Golden Staph’ or ‘Multi-drug resistant Staphs’. Chemotherapeutics, particularly antimicrobials, should NOT be used as prophylaxis but rather reserved for treatment purposes. Otherwise, you will run into drug resistance problems. Just imagine how unfortunate it would be if one day one of your fish gets sick and there are no more drugs that are effective left. What is of greater concern (especially with regard to antibiotics) is if these resistant bacteria pass on their resistance genes to bugs that can cause disease in humans, how can we treat ourselves? We need to take responsibility and consider the long term implications.

Drugs vary in efficacy and toxicity in different conditions and may even be toxic to certain species, e.g. Malachite green. Malachite green is toxic to scaleless fish, certain catfish and clown loaches. It is more toxic in acid conditions and at higher temperatures. And for this reason, I am a little wary of using multi-drug remedies usually marketed as a ‘cure-all’. Quite a few medications on sale do not state what chemicals are in the bottle and you could run into problems. Malachite green is also a carcinogen (it increases the risk of cancer) and the operator must practice caution to avoid contact with the chemical.

Sometimes topical or bath treatments are ineffective. If the fish is still eating, we can get drugs into them via medicated feed (which can be compounded to deliver the require dose) or by intramuscular injection if your fish has gone off food.

A final note about medications and fish. Fish are classified in the pharmaceutical world as ‘minor species’. This means that they are not a significant part of the drug market and so there are not that many drugs available that are registered for use in fish. Non-registered products can still be very effective and safe for use in many animals but drug companies tend not to register them mainly because of the extra costs involved. Veterinarians are permitted to use and prescribe drugs ‘off-label’ in accordance with State legislation.


So, to summarise the main points:

  1. You need the correct diagnosis in order to prescribe the right medication, if not you may do more harm than good;
  2. Shotgun therapy is risky for the fish and contributes to bugs becoming resistant to drugs – wasting your time and money;
  3. The right diagnosis means you need specialised equipment and a trained diagnostician with experience and knowledge of fish.
Contributors: Dr Matt Landos BVSc (Hons) MANZCVS  
By Dr Richmond Loh – Last updated 16 November 2012
Share this article

Related articles