Fish health

More recently, there has been a movement against using the phrase “fish diseases” in favour of “fish health”. I would suggest this is rightly so, since “fish diseases” is a negative phrase implying that it is almost inevitable for fish to contract an ailment.  It also deflects attention away from the fact that, as caring hobbyists, we should be providing the best possible conditions for a prevention against such diseases. Bearing all this in mind, it has to be said that marine fish are far more likely to experience some form of ailment than their freshwater counterparts, mainly due to the fact that the marine environment is so much more difficult to maintain.

Preventing Disease

Preventing disease is obviously far more desirable than having to treat a sick fish whereas an infected aquarium is going to need special care and medications. The main causes of marine fish ill-health fall into three categories: overstocking, overfeeding and stress. It could be argued that the last category occurs as a direct result of the first two, but there are many other unrelated causes of stress including bullying, bad handling, poor compatibility with other tank mates, etc. What then are the best ways to prevent disease?


Always stock slowly. The quicker a tank is stocked, the greater the risk of disease.  This is especially in a newly matured tank.


Feed no more than the fish can consume in a few minutes. Remove any excess immediately.  There are very few instances of fish dying because they are underfed, so do not be afraid to feed sparingly.


Marine fishes are easily stressed.  This can ultimately lead to a rapid breakdown of their natural immune system and the subsequent contraction of disease. Apart from the causes of stress already mentioned, stress can be brought about by high nitrate levels, unstable water conditions, loud external noises (slamming doors, loud music, tapping on the glass, etc.) and toxic fumes within the room (heavy smoking, carpet cleaners, paint fumes, insecticides, etc.).

Early Signs

Nearly all diseases will be signalled, at an early stage, by a change in the behavior of the fish. A depressed appetite is usually a clear sign.  This is as well as hiding or sudden shyness. This may be accompanied by more dramatic symptoms such as flicking or scratching against rocks, swimming erratically, rushing around the tank in a frenzy or an inability to maintain swimming equilibrium. A good fishkeeper will know the normal behavior of the fish and recognize these early warning signs so that swift action can be taken.

Quick action is important as many marine diseases can become dangerous within 24 hours making treatment progressively more difficult.


There are four main types of diseases: parasitic; bacterial; fungal; and viral. Often, one disease will complicate another. It is not often discussed, but fish can suffer from most of the same physiological diseases as humans or other animals. Heart, kidney, liver and intestinal problems are bound to occur from time to time.  This can be alongside a whole host of other disorders. The point to bear in mind is that if a fish is ill, it may not be one of the common aquarium diseases.  Sometimes there is no effective treatment and a distressed fish may have to be destroyed humanely. If in doubt, seek expert advice from a veterinarian specializing in marine fish.

Common marine aquarium diseases

marine sick fish

Also known as Cryptocaryon irritans.

Symptoms include flicking and scratching against various surfaces and small (1mm) white spots covering fins and body.

Whitespot is a highly infectious disease that spreads rapidly and needs an equally rapid medicinal response. Treatment is the use of copper-based medications which will kill the vulnerable free-swimming stages of the parasitic life cycle. Using an ultraviolet sterilizer in the tank can help to control the disease at the same stage.

Oodinium or Marine Velvet. Amyloodinium ocellatum

caring for your fish

Symptoms include flicking and scratching against surfaces, rapid breathing and very small spots peppering the body producing a “velvet” appearance in the worst cases.

Perhaps the most infectious and deadly of all marine diseases. Swift treatment is essential.  Some strains of this disease can be extremely resistant to medication. Treatment is the use of a proprietary copper-based medication to kill the vulnerable free-swimming stages of the parasitic life cycle. Once again, using an ultraviolet sterilizer in the tank will destroy the free-swimming parasites.

Black Spot

Symptoms include small (1mm) black spots on the body. Fish will flick and scratch.

The spots are not usually so numerous as those connected with whitespot disease, neither are they so deadly. Surgeonfish seem to be particularly vulnerable.

Treatment is the use of a proprietary copper-based medication or one based on Trichlorofon.

Gill and Fin Flukes

Symptoms include rapid breathing, flicking and scratching, cloudy eyes and white patches on the skin.  Worm-like attachments are sometimes visible but not always.

Another very infectious disease that can quickly become fatal.  The fish will suffocate if the gills are infected.

Treatment is the use of a freshwater bath for immediate relief and proprietary anti-fluke medications.

Head and Lateral Line Erosion Disease (HLLE)

aquarium fish care

Symptoms include the erosion of the lateral line and the formation of pits in the skin.  This disease is very similar to freshwater hole-in-the-head disease.

The condition of the fish gradually deteriorates and often results in the death of the fish.  Consequently, the water conditions must be vastly improved.

There is no direct treatment for this disease.  No real proprietary cure. This disease is a direct result of poor environmental conditions.  You will need to investigate and improve the environment.


caring marine aquarium fish

Symptoms include the erosion of fins and fin rays.  Reddened areas, particularly at the base of fins. Lethargy and depressed appetite. In very bad cases the body of the fish can also begin to rot as well.

Treatment includes immediate improvement of environmental conditions proprietary bacterial medications.

Marine Fungus

Ichthyophorms spp.

Symptoms include fish darken markedly, depressed appetite, listlessness and sandpaper appearance of the skin.

This is not particularly common and requires careful diagnosis but is a good example of a fungal disease.

Treatment is very difficult, as proprietary antifungal medications are not always effective. Food soaked in Phenoxetol can be effective if the fish is eating. Improve environment.

Cauliflower Disease Lymphocystis spp.

betta fish Cauliflower Disease

Symptoms include warty clumps of miniature “cauliflowers” which form on the fins and body.

This viral disease often looks a great deal worse than it really is. Although rarely fatal, secondary infections may occur. Treatment: there is no proprietary medication. A freshwater dip may help. The fish's own immune system will eventually defeat the virus.  This is providing optimum water conditions are maintained and the fish is under very little stress.

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