How Can You Diagnose Fish Disease?

Reading marine aquarium potential disease pages in a book could be enough to make an aquarist’s hair stand on end! Various problems appear to be lurking ready to inflict doom on the unsuspecting livestock.

It doesn’t ever have to be a problem – disease that is. The advance in collection methods and aquarium husbandry has been high over the last decade and there must be many aquarists who have never faced disease problems. It must be said though that the problems could still strike. It has been stated many times that the major defence is with the aquarist, from the moment he/she goes into the LFS to the moment the fish is released in the display aquarium observation and proper procedures should be observed. Even if all is as it should be, there remains the possibility of problems.

The good news is that most diseases can be dealt with successfully nowadays if the disease can be attacked. The bad news is relevant to the latter part of the last sentence. For example, copper is a prime treatment of certain parasitic problems and copper is also deadly to invertebrates. So if the aquarist runs a reef system copper is not the choice treatment.

Observation of fish stock should be a daily occurrence. This is easily achieved as the aquarist will usually feed daily and the fish are easily seen. If one doesn’t come out to feed there isn’t a need to immediately think there is a problem, but a warning bell should ring. Next time, is the fish there? If not, what’s going on?

If fish are seen to be swimming strangely such as darting about, apparently unable to hold a normal position in the seawater, lethargic, breathing more quickly than usual, or rubbing (flicking) against rocks then the aquarist should first of all test the seawater. Do an ammonia and nitrite test, followed by a pH test. If all is normal and the seawater is of its usual high quality then move on. If there is something wrong with the seawater then deal with it – a larger than usual seawater change or changes will dilute any ammonia or nitrite presence. Make sure nothing has happened to damage the bio-filter, for example if a canister filter is in use they are driven by an electric motor which could have slowed or stopped. If the pH is low is the seawater circulation as it should be? If it isn’t then oxygen intake could have been affected. Fix the offending article such as a powerhead etc.

So overall the seawater is the first suspect and it will not take much time to confirm this one way or another. If there isn’t a problem with the seawater then attention returns to the fish.

Correct disease identification is very important. It will not be of much use applying medication if it is the wrong sort. Also, applying a wide range of medications in the hope that one of them will do the trick is not a good idea; it will cause additional stress to the fish and probably exacerbate the situation. The aquarist should carefully observe the fish and try to see if there is anything unusual about them. There could be cloudy eyes, torn fins or little white/grey dots on the fish etc.

Identification of potential diseases of fish is a large and detailed subject and not suitable for this text. As has been said before there isn’t any need for an aquarist to be a scientist or engineer, but there is a requirement for the basics to be understood.

One of these basics is to be prepared, like a good scout. So what there should be, along with the test kits etc is a good marine aquarium book. Usually it isn’t a great deal of help going on the internet as to obtain relevant information the disease identity needs to be known. Asking for assistance and advice on a forum is good normally but in this case there is likely to be more than one identity suggestion as some disease indications are relevant to more than one problem, and in addition time is passing. Some problems need reasonably speedy action.

Picking a suitable book is easy as content can be ascertained in advance. The bonus is that there will be a lot of information relevant to the marine hobby other than diseases, so the aquarist will have a source of information literally at his/her finger tips.

So basically the routine is:

Observation – should be the habit daily.
Seawater – is the quality as it should be? Test and find out.
Identification = the most difficult. Having a good book which contains a diseases section is the best way forward. It will also detail suggested remedies.

On this website ( are texts which are relevant to fish diseases:

Click on ‘Articles’ (top of page) then click at left hand side on ‘Aquarium Care’. Then click the article ‘Controlling Fish Tank Diseases.’

Again on ‘Articles’ and again under ‘Aquarium Care’, click on ‘An Aquaristic Nightmare.’

Click on ‘Blog’ (top of page). Then click on ‘Livestock’ then on ‘Fish’. Scroll down, there are some relevant to disease.