Why aquarium fish die......



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Aquarium Doctor

Why aquarium fish die......

Normally, an alert aquarist can see the symptoms (frayed fins, cloudy eyes, unusual behavior, water test results) of a problem before a death occurs, but ocassionally a fish will die suddenly with no apparent reason. Barring the possibility of a fish becoming infected and dying from the West Nile Virus, below are the most probable reasons for a tank critter to die.

The Nitrogen Cycle

One characteristic, next to passion for the hobby, every aquarist should have or obtain, is patience.

Patience, next to understanding the basic water parameters, will be put to the test while cycling a tank. And cycling by all means, not only during the fresh set up of a new tank. An established aquarium can cycle at any time, depending on severe changes of the bioload, filtration failure, or any loss of nitrifying bacteria, or adding new animals.

The cycling process starts the aquarium. Since an aquarium is an artificial and fragile ecosystem it requires our “interference” in order to thrive. Our interference starts with providing an “artificial” filtration system. In short, creating an environment as close to nature as possible.

"nitrosomonas bacteria"

This is the most important life in your aquarium! 

Requiring oxygen to survive. Many bacteria must have a suitable supply of oxygen to be able to survive and thrive. Bacteria such as nitrosomonas and nitrobacter are aerobic and must be supplied with a constant flow of oxygen in the water to create suitable populations able to remove the ammonia and its by-products produced within the aquarium.

Ammonia Poisoning

Occurs most frequently while cycling a new tank, but also happens during collection, while in transit or adding a new animal.

If the tank has been fully cycled for a period of time, New Tank Syndrome may be the cause.

Symptoms include cloudy eyes, frayed fins, rapid gilling and lack of appetite.

Nitrite Poisoning

Almost always occurs while cycling a tank.

The symptoms are similar to ammonia poisoning.

Cyanide Poisoning

Almost exclusively occurs in fish captured in certain areas of the South Pacific.

May take a week or two to show up after collection.

Other than a lack of appetite, there are few symptoms to detect.

Poisonous Sting

May be inflicted from a poisonous fish(i.e. Lionfish) or from a poisonous anemone.

Difficult to diagnose.

Symptoms include erratic behavior and lack of appetite.

Other Poisons

Poisons from the Neutron Bomb Boxfish, Pufferfish or Lionfish which have been released in the tank.

Anything from cigarette ashes to hair spray can be sources.


Most often occurs when a new fish is introduced into a tank.

Lack of appetite due to disease.

First symptom is a sunken belly.

Oxygen Deprivation

Normally caused by lack of vertical water movement in a tank. (Fish need aeration from air pump).

Initial sign is fish staying at the surface of the tank water.

Old Age


Difficult to diagnose.

Physical Injuries

Pretty obvious.

Usually caused by other tank occupants.

Bacterial Infection

Symptoms include cloudy eyes, red areas on the body, swollen belly (internal infection).

Bends (Diver's Disease)

Rare for the hobbyist to see.

Normally occurs during collection.

Other Diseases

Ick, fungal infection.

What information do we need? Ick=Small white pin head spots, fungal=white fuss

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

Table 1: Basic health work-up Investigation


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

More Information

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

More Information

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

More Information

9. Water tests for nitrate, phosphate and dissolved organics

These may indicate background pollution, poor maintenance or inadequate filtration

More Information

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

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

More Information

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. See anesthetics pages

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.

More Information

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 out your own fish health work-up form

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.

The Nitrogen Cycle

"nitrosomonas bacteria"

This is the most important life in your aquarium! 

Requiring oxygen to survive. Many bacteria must have a suitable supply of oxygen to be able to survive and thrive. Bacteria such as nitrosomonas and nitrobacter are aerobic and must be supplied with a constant flow of oxygen in the water to create suitable populations able to remove the ammonia and its by-products produced within the aquarium.

AQUAPAGES Aquarium Doctor

In this part of the website, you will also find out how to take care of algae problems, what to feed your fish, what the proper water parameters are, and many other good techniques to keeping your fish and invertebrate happy and healthy. With this information, members will have a much better understanding of keeping their tank.


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