Help! My Fish Are Unwell

What can I do if my fish are sick or dying?

Water quality?

Many fish health issues are related to stress due to a water quality problem. Treating fish with a medication is unlikely to prove of much lasting benefit unless any underlying water quality issues are also dealt with. If a large number of fish have suddenly started to behave unusually (e.g. hanging or mouthing at the water surface), then suspect a water quality problem to be at least partly to blame.

Oxygen levels

You can boost oxygen levels in the water by turning on any pumps that disturb the water surface, such as fountains, waterfalls or air-pumps. This is especially important in hot weather when pumps are best run 24hrs a day. Remember that underwater “oxygenating” plants only produce oxygen during daylight hours. In ponds without pumps, see the page on “Help, there is a power failure” for advice on boosting pond oxygen levels.

Pond filters

Biological pond filters can greatly improve water quality when they are functioning correctly. However, if their activity has been disrupted by becoming blocked, or by a power cut, or by a strong pond treatment chemical, then water quality may drop rapidly. Dirty or clogged filters should be cleaned according to the manufacturer’s advice, using pond water - not tap water. Remember that biological filters are best run 24hrs a day throughout the season in our UK climate.

Feeding fish

If fish have inexplicably gone off their food, this can be another sign of a problem. Even if they are still feeding, trim back on the amount of food fed until the problem is sorted, and be sure to net out any uneaten food.

What to look for

If one or two fish are ill, look out for specific symptoms to aid in diagnosis. Is there any obvious damage to the skin or tail, e.g. from heron or cat attack? Are the sick fish beating their gills at an abnormally fast or slow rate? Is the skin surface unusually hazy in appearance? Are the scales or eyes protruding? Are affected fish hanging beside the waterfall or fountain? Are fish scratching/flicking onto their sides with unusual frequency?

If you take a little time to watch your fish when you are in the garden, you will get to know their normal behaviour. It is then easier to spot any unusual behaviour that might suggest a problem.

One fish, losing its balance

Pondfish have small air-sacs inside them, called the swim bladder. This helps them to maintain their buoyancy in the water. In some cases this sac can become damaged or distorted, and affected fish then have to depend on moving their fins to maintain their balance. This causes more problems in the winter when fish are sluggish and have less energy to use their fins. Affected fish may tend to float or sink, or roll at an angle, and may appear to struggle to keep their balance. The problem sometimes disappears in warmer weather, only to reappear the following winter. Swim bladder problems are more common in the fancy types of goldfish with shorter bodies, and are not normally treatable. Some affected fish may live for many years without coming to harm, while others might be more vulnerable to infections.


When problems arise, in existing or new fish, shortly after adding new fish, this might be due to the new fish introducing an infection, or it could be due to the new fish being stressed because the water quality in your pond is different to what they were used to, or it could be that the new fish are being affected by an infection that your existing fish carry but have become immune to. It could also be that the increased stock of fish is overwhelming your pond’s capacity, resulting in a drop in water quality that is stressing all the fish. Further investigation is recommended before coming to conclusions. Speak to your fish supplier first to see if they can help. www.watergardensolutions.co.uk


Whilst fish problems are occurring, do not add any new fish to the pool. When purchasing fish for your pond, buy only from reputable suppliers (e.g. members of a trade association such as OATA) who routinely acclimatise and health-check their new fish, and who can offer support and back-up if you have problems with any new fish purchases. Do not buy fish from tanks where other fish appear unwell. It is always preferable to purchase from a site that you can visit, so you can check the quality of the fish personally before buying.

Take care to build up fish stocks gradually in a new pond, and check that your pond is suited to the fish varieties chosen (e.g. koi will require larger ponds which should ideally be filtered to a high standard; Sturgeon and Sterlet have specialist requirements and are not suited to smaller ponds). Always avoid overstocking.

What to do next

Testing the water quality, and closely examining fish can often pinpoint the cause of any problem. If skin parasite problems and minor bacterial infections are caught early enough, they can usually be treated very successfully. More severe bacterial infections such as ulcers can prove difficult to control.

In our local area we can visit and carry out the necessary water tests and microscopic examinations as required. Contact Us to discuss any issues you are concerned about or to arrange an on-site consultation.

Useful links:

Find a local fish supplier who might be able to offer fish health advice and water treatments

Find a vet who can deal with fish health issues

See more Frequently Asked Questions

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