Fish Disease Diagnosis

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Don’t just guess what the problem is!

Guess or diagnosis?

Let me say right from the start that it is just not possible to diagnose fish disease and health problems without first examining the affected fish and possibly the fish’s environment. After all, would you trust a doctor who told you what was wrong with you without asking any questions or carrying out any examination?

Would you phone up your vet and expect him to tell you over the telephone what was wrong with the family cat or dog?

Obviously the answer to both these questions is no.

Trying to guess what is wrong, (and that is all that people do if they don’t carry out any investigation), is obviously the wrong way to go about things, unless of course you believe that aquarists and dealers have some form of mystical powers not possessed by vets and doctors!  Common sense and our own experience tell us that the right way to proceed is to establish what is wrong and why, before we carry out any treatments.

Detective work

It is important to realise that fish are aquatic animals and can be affected by most of the diseases that typically affect other animals, as well as diseases and health problems that are exclusive to an aquatic environment. However, unlike most terrestrial animals, they have a limited response to disease. For example, you will not see a fish holding up a sore fin, or limping or even whimpering. Their response is limited to refusing food, lethargy, jumping, rubbing or flashing, gasping at the surface and isolating themselves. There may be minor variations, but essentially these are the only ways that they can let us know there is something amiss.

The main point is that they will exhibit some of these signs irrespective of what the problem is. So we see the same responses to poor water quality, gill disease, parasites, heart problems etc. Given that they will respond in much the same way, irrespective of the cause, it is simply impossible to make a definite diagnosis based purely on behaviour.

Trying to determine what is causing the problem is a little like detective work as there are so many probable causes. Behaviour is just one small part of the puzzle. The initial aim in diagnosing health problems is to collect as much information as possible. When we have this information we can then make a balanced assessment of both the problem and the likely cause.

What information do we need?

Table 1 shows the most basic necessary information needed when investigating health problems:

Table 1: Basic  health work-up



1. Size of pond / tank. Number and size of fish and type of filtration system This tells us stocking densities and whether the filtration is adequate. As stocking densities increase water management and filtration become more important

2. Any new additions. Any treatments carried out in last 4 weeks This may indicate a transmitted disease, or a toxicity problem related to disease treatments. 
3. Behaviour over the last few weeks and days This may suggest the possibility of certain health problems such as water quality, parasites etc. 
4. Are the problems affecting all the fish or just one or two individuals If more than one or two are affected it will probably indicate a contagious condition and / or environmental problems
5.  Have they been suddenly affected within a short period – usually days? An acute problem usually indicates a serious water quality problem or poisoning, especially if an examination shows no other disease signs
6. Are the problems chronic – an ongoing problem affecting  them over a period of a few weeks This could indicate parasites, bacterial problems or a background water quality problem
7. An examination in the water to check behaviour, respiratory rate and any obvious physical damage This gives a very basic indication of what sort of disease problems might exist

8. Carry out water quality tests for ammonia, nitrite, pH, water hardness and history if available These may indicate a core water quality problem or toxic conditions

9. Water tests for nitrate, phosphate and dissolved organics These may indicate background pollution,  poor maintenance or inadequate filtration

10. Examine  system, including filter,  for water clarity, algal growth, solid wastes:


This may indicate possible pollution sources; whether photosynthesis is affecting pH or oxygen levels and whether the system is poorly maintained

11. Examine a fish out of the water, usually while it is sedated. It is not possible to examine a fish while it is still in the water. Check skin, fins, eyes, mouth, body and gills We are looking for lesions, reddening of the skin, fin-rot, visible parasites, the colour and condition of the gills and any other visible signs of disease. 
12. A skin scrape, during which a small amount of mucus is carefully removed with the back of a scalpel or wooden spatula, so as not to damage the epithelium. The sample is then examined under a microscope.  This examination will show whether parasites are present; what sort and how many. The best sites for sampling are just behind the operculum and along the back at the base of the dorsal fin.


13. A gill biopsy or gill swab should be taken. The biopsy or swab is then examined under a microscope


This will show whether parasites are present in the gill; what sort and how many. It will also give some indication of the condition of the gill.

14. If possible take a fecal sample for microscopic examination It may show signs of internal parasites
Print a Fish Health Assessment Guide
With all of this information regarding the physical condition of the fish and the environmental conditions it is, in most cases, possible to determine both disease and the cause. From this we can decide what actions and what treatments are needed.

If this basic examination is inconclusive it may be necessary to carry out further investigation which could involve post mortem investigations of recently dead fish, bacterial sampling from lesions to determine the type of bacterium involved and their antibiotic sensitivity, histological examination which involves preparation and examination of body tissues and organs for signs of malfunction and disease.

Although this may all look involved and unnecessary the success rate of this type of methodical approach to fish disease diagnosis is far, far higher and usually a lot cheaper than either guessing or making simplistic diagnosis that only treat the obvious effects and not the cause.