Fish Diseases and Ailments

Just like us, fish suffer from their share of health problems and there are few things more alarming than noticing that some of your prized pond life is looking ill – or, worse still, dying.

A range of disease causing agents are commonly found in ponds, but fortunately many of them don’t cause much trouble for fish that are generally healthy and any outbreak can be treated relatively simply. There are others, however, which pose a much more serious threat, making any delay in diagnosis potentially disastrous. Clearly then, it pays to have a good working knowledge of the diseases and ailments likely to affect your fish – and to keep them well fed and in good condition.

Spotting the Signs

Many fish ailments can spread at an alarming rate if left untreated, so forewarned is definitely forearmed, so it’s essential to be able to spot the signs that all is not well at the earliest possible opportunity. Healthy fish have clear, bright eyes, upright fins and swim easily and well; any that don’t should be examined more closely, paying particular attention to any cuts, ulcers, lumps, cloudy eyes, fungus or missing scales. Poor feeding, sinking to the bottom or floating to the surface are also signs that call for further investigation.


Parasites cause a number of common fish problems, and the damage that some of them do can make affected specimens more susceptible to secondary infections. Three of the more serious ones to keep an eye out for are:

  • Argulus (Fish Louse) – large numbers of this blood-sucking parasite can seriously weaken a fish; the first symptoms are usually a fish rubbing itself in an attempt to shake off its unwanted guest. Argulus grows to up 1cm, so spotting it isn’t hard.
  • Ichthyophthirius multifiliis (White Spot) – a serious threat, which can cause major tissue damage and widespread fatalities if left to its own devices. Infected fish are lethargic and anorexic; careful examination reveals small white pimples the size of pin-heads on their bodies, fins, eyes and gills.
  • Lernaea (Anchor worm) – it’s actually a crustacean, not a worm which penetrates deeply through the skin and into the underlying muscle; another parasite that is visible to the naked eye, Lernaea, particularly the females, which have two elongated egg-sacs that typically lie alongside the body of infested fish.

Fungal Infections A fungal infection normally looks like a fluffy growth of cotton wool on the body of the infected fish – and it can spread at an incredible speed. Damaged skin is usually the initial cause, particularly if nitrite or ammonia levels are relatively high in the pond water. Fortunately, treatments for fungal disease are widely available and they are normally successful, and particularly if they are used in the early stages of infection.

Bacterial Infections

Bacteria are ever-present, but they tend to make a particular nuisance of themselves when fish have first been debilitated by stress or disease. Fin rot is especially prevalent under these circumstances when bacteria, which are naturally associated with fish, multiply to such large numbers that they begin to erode the soft flesh of the fins and tail, leaving the bones protruding from the frayed connective tissue. If left untreated, the infection can track back to the skin and muscle, so it should always be dealt with as a matter of priority, once spotted.


Ulcers often appear at the site of physical damage, when secondary infections take hold, and usually only one or two fishes will be affected – though if more seem to be suffering, it’s worth investigating further to see what’s going on. Although it can look unsightly, the natural healing ability of fish will often deal with minor cases of ulceration and there are a number of proprietary treatments available – but professional help may be necessary if things show no signs of clearing up quickly.

Water Quality

It has been said that bad husbandry accounts for around 90 per cent of a fish pond’s health problems and water quality is arguably the single most important factor, principally because it can be a major stressor of otherwise perfectly healthy animals. A stressed fish soon becomes an immune-compromised one, opening the door to a range of infections and infestations that can drag it down and may ultimately even prove fatal. It should go without saying that staying on top of your pond’s water quality is an essential part of safeguarding the health of your fish – so test, test and then test again!

Keeping your fish in good health calls for a high degree of vigilance, coupled with swift and decisive action once a problem has been detected. The good news is, however, that most fishy ailments can be treated successfully, if they’re spotted early enough – so just keep your eyes peeled for trouble, and all should be well.

Last Modified: May 24, 2022