Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Causes of fish health problems

Neon tetra, Paracheirodon innesi
Neon tetra, Paracheirodon innesi

So what is stress exactly?

Stress occurs when a fish is placed in an environment which is beyond it's normal tolerance level in some way. This may include one or more of the following:

Poor water quality

When fish show signs of illness the water quality is one of the very first things to check. Quite often poor water quality is at the root of the illness and by using a variety of water tests we can quickly find the cause of the illness. Even if the water quality turns out to be fine we will have eliminated many of the potential causes of the illness.

What do we mean by poor water quality?

Well it doesn't necessarily mean polluted water, it simply means that the water isn't suitable for a particular species. for example a discus fish wouldn't do very well in a tank set up for Tanganyikan cichlids because the water would be far to hard, the pH would be far to high and the temperature to cool all of which would cause stress and quickly lead to ill health and possibly death. There are other things to be aware of too. Is there enough dissolved oxygen in the water, if the tank is kept to warm and it is over crowded then the chances are it will be low in oxygen.

Is there a build up of nitrogenous waste?

If there is this too could mean low oxygen because water has a finite capacity when it comes to having things dissolved in it.


Although less likely than the other causes it can still happen, some possible causes are aerosols, insect spray, young children adding something to the tank (it happens), smoke and so on.

Poor environment

Does the decor have sharp edges?
Are the stones safe to use? Might they contain calcium or magnesium both of which will make the water hard and possibly raise the pH.
Is the wood safe to use? Might it still be green and contain toxic sap or has it been sprayed with insecticides or fungicides?
Is the aquarium set up appropriately for the species being kept?
Is the lighting to bright, are there sufficient and suitable hiding places, is the aquarium big enough.
Is the aquarium badly placed? Near to a door, next to a TV, next to a radiator or in direct sunlight.
All these things and more will lead to the fishes becoming stressed.


Should be avoided at all costs, if a filter fails as they occasionally do the water quality will go down much faster in an over stocked tank than in a sensibly stocked tank.

Non shoaling species will feel crowded and may react aggressively. Some of the quieter species may lose out to the bolder species at feeding time, nitrogenous waste will build up faster, disease will spread more easily.

Crowding DOES NOT reduce aggression, it may do where Malawi cichlids are concerned but there are valid reasons why that is the case and there are valid reasons why it won't work universally.

There are no advantages to be found by over crowding, it simply is likely to lead to problems.

Aggressive tank mates

Would you like to be locked in a room with a large aggressive dog? No well do you suppose the any fish will much enjoy sharing their tank with a larger and more aggressive fish which is intent on biting them.

Apart from the obvious risk of injury it would be extremely stressful and the unhappy victim isn't likely to last very long without intervention.

If you come across this due to a rogue fish which is meant to be peaceful but... You must act immediately to remove the aggressive fish. Don't wait to see if it settles down because doing so may mean losing one or more fish to bullying.

Raised bacterial levels

If an aquarium is neglected, over fed or over stocked the filter may not be able to keep up. A build up of organic waste could lead to a steep rise in normally harmless background bacteria numbers which could then become pathogenic and begin to overwhelm the defences of stressed fish.

If an infected fish is is introduced to an aquarium it will already be be hosting high levels of bacteria, in an unsanitary aquarium with stressed fish the infection is more likely to spread than in an healthy system.

Wrong Temperature, incorrect diet poor handling and external disturbances are other causes of stress for fish.

Ok so we know what stress is and what causes it,
but what does it actually do?

The first thing that happens when a fish is placed under stress is a rise in blood sugar caused by hormones released from the adrenal gland. This prepares the fish for the flight or fight response.

Osmoregulation is disrupted due to changes in the mineral metabolism, fresh water fish will then absorb excess water and marine fish will lose to much water. Both respiration and heart rate increase and extra red blood cells are released in to the blood stream.

A fish can only keep up these changes for a short time before they start to use up vital reserves of energy and exhaust the fish, some of the changes if they last for a long time will actually harm the fish or even cause it's demise.

Document CIR919, Fisheries and Aquatic Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.

General fish health

Fish are naturally quite resistant to diseases but if they are under some sort of stress then this situation can easily change. In captivity there are all sorts of things which can add to the stress that fish have to live under but because fish has very strong instincts to act normally regardless of how it actually feels it is easy to assume that everything is fine.