Koi pond fish stress

Fishkeeping should be available on prescription from the NHS. Scientifically proven to cause a drop in blood pressure and reduce heart rate, watching fish should be recommended to anyone who feels they need some stress relief.

Be it lounging by a pool, lapping up the gentle movements of fish weaving hypnotically in and out of plants in their submerged garden, or drifting in and out of consciousness listening to a babbling waterfall or the mesmerising patter of a fountain, fish are a must for relaxation.

Stress in our busy lives is on the increase and has become a significant factor in the UK economy, surprisingly though, stress is also a key problem for fish themselves and is recognised as the No.1 killer of fish. Who’d have thought it, that our finned stress-relievers could themselves suffer so widely from stress?

What is stress?

Stress experienced by fish is the result of a prolonged series of chemical changes that are brought about by fish experiencing a stressor that is causing a negative impact on that fish.

Fish are well adapted for escaping from short term pressures (such as predatory attack or us trying to catch them with a net – they respond to both in the same way) and will generally take a good number of hours to recover from such an ordeal. Yet if the fish are subjected to a prolonged stressor then the fish’s response is such that it inadvertently fish becomes far more susceptible to disease. A long term stressor could be a gradual deterioration in water quality or the constant chasing or competition from other fish.

The key consideration then is to keep fish stress-free or we will become stressed ourselves by treating problems encountered as a result. Our fish will not reward us with vivid colours, healthy growth and tame behaviour but burden us with conditions or symptoms that will need diagnosing or treating.

Stress can occur in places other than a garden pond. Consider the factors when buying a fish.

The actions of netting, bagging, carrying, and transporting fish are all foreign to the fish and will be responded to in a stress response. We may well know that our new acquisitions are going to a spacious and pampered pond, but the fish does not realise that, nor does it recognise that it’s journey may perhaps only take 1 hour. From the start, a fish’s body is on auto pilot for survival undergoing physiological changes that would save it under a short term stress in the wild, but cause havoc with its immune system as a result of the long term stress.

How can I recognise stressed fish?

Hopefully, a stressed fish will never be an occurrence in your pond, but if it is, it must be identified early before a fish’s susceptibility to disease increases significantly and to be able to rectify the problem and eliminate the stressor as soon as possible.

‘Fish watching’ should be an integral part of pondkeeping. The behaviour of your fish can be used by an experienced pondkeeper as a barometer of the pond conditions. Putting it simply, an unexplained change in fish behaviour should be the first sign that your fish are being affected by something in the environment, to which they are responding with ‘escape’ or other abnormal behaviour.

Typical stress responses:-

1. Hanging at surface, near waterfalls or fast flowing water.

This suggests a low dissolved oxygen concentration in the water perhaps as a result of overfeeding, a reduced turnover through a blocked pump or filter, hot and clammy weather or the side effect after chemically treating the water.

Remedy: Improve pond water circulation, aeration and perhaps carry out a partial water change.

2. Sudden loss of tame behaviour

Fish no longer eagerly feed at the surface, perhaps even ignoring food altogether. In extreme cases it may be difficult to catch sight of all of the fish as they are so reluctant to leave the cover of the pond bottom. This may be caused through the attentions of a heron or cat, where the stress response is to ‘hide’ from the predator. However, a drop in temperature could also have cause this behaviour which is quite an innocent reason for a change in fish behaviour.

Did You Know?

Certain sudden changes in a fish’s environment are natural and are relied upon as stimuli to promote spawning activity in pond fish. These may include sudden seasonal changes in temperature and water quality.

3. Fish feed but with less vigour than normal. Overall behaviour seems a little ‘sluggish’.

This may be through a build up of toxic fish waste that has not been broken down effectively by a biofilter. Such a build up is toxic to fish and hinders fish from excreting further toxic by-products from their body.


Test water to confirm an unacceptable build up of ammonia or nitrite and carry out a partial water change accordingly. Ensure effective filtration prevents such a problem in the future.

Prolonged stressors in a pond can affect fish steadily over time, where the build up is gradual and perhaps unnoticed. In addition to a change in behaviour, other responses to look out for can include:

a) Paler areas of skin turning ‘pink’ through the increased appearance of fine blood capillaries in the fish’s skin.

b) Excess mucus production and associated dulling of colours is also a good indication of a stressing water quality problem.

c) Increased breathing rate with fish actively ventilating their gills to gain extra oxygen.

Factors that can lead to stress can be classed as acute (short term changes) or chronic (long term changes).

Acute Chronic


Build up of nitrates through a lack of water changes

Chasing with a net

Being chased or out-competed at feeding time

Low dissolved oxygen levels

Gradual spread of plants choking pond of space and oxygen at night.

Overfeeding leading to poor water quality

Deficient diet reducing health and vigour over time.

Heron/Cat attack

Harsh cracking of ice

As seen in the table, the majority of stressors in a pond are preventable and it is in our own interests to do just that. There is very rarely such a thing as a fish dying of natural causes in garden ponds as the majority of fish losses can be traced back to some stressful human interaction. Take full notice of what your fish are telling you through their behaviour and respond swiftly to any changes to prevent the unnecessary stress on your fish and yourself. Keep fishkeeping a stress-free pastime.

Kill blanketweed and string algae.