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Fish Diseases and Ailments

By: Dr Gareth Evans - Updated: 31 Oct 2020 | comments*Discuss
Fish Illness Health Ponds Water Quality

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 InfectionsA 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.

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I have some grass carp and bream i have had them about five years but are starting to get deformed shapes like a S what is the course of this ?
barney - 31-Oct-20 @ 12:16 PM
Have a well established pond, lastyear we had 15 new babies all black, this spring one or two have skin blemishes around the head and gill areas, i cannot identify these blemishes, so treating the whole pond with a general treatment blue. Could this be skin or pigment changes showing after 2 year growth.
Reg - 16-May-20 @ 12:56 PM
I have a fish with the same problem as Probie, laying on the bottom,very bloated then swimming around in circles. What can I do to help the fish?
Wisemaiden - 3-Aug-19 @ 4:35 PM
I have the same problem as Probie 23rd March, I have a fish doing the same , very bloated, laying on the bottom, then swimming around in circles. What can I do to help or put right the problem?
Wisemaiden - 3-Aug-19 @ 4:30 PM
One of my fish in my pond is severely bloated, "potbellied." I can see it sometimes laying at the bottom of the water on it's side or upside down, and other times it swims around as if nothing is wrong. Should I euthenase it or leave be?
Probie - 23-Mar-18 @ 2:56 PM
Like Lyndy 2JunOur fish are dying with no visible signs of anything wrong.The water is clear, ph fine,Any ideaswhat could be causing it?
Linjay - 3-Sep-17 @ 4:41 PM
Hello, I have a new pond - its just 4 months old and its wildlife friendly ie no fish or chemicals are used. Whilst in it in bare feet, I realised there were leaches - tiny 1cm long. I also found a 10cm long yellow newt which had three or so of these leaches attached to it. I have trawled the internet and cannot seem to find any info on leaches in a garden pond. Will the leaches be eaten by the larvae of the many dragonflies which have laid their eggs in the pond? Will they harm or kill the newt? Many thanks for your help. Rexanna
Rexanna - 21-Jul-16 @ 6:43 PM
We have a very old pond (8x6 foot and 1.5 foot deep).It has been there for over 200 years.About a week ago our fish (about 12) suddenly started dying.The water circulation system had stopped working about 2 weeks prior so we thought it may be this.We managed to get the circulating system of the water back up and running and bought a pump with airiating stones on it and added a treatment (Blagdon Anit Ulcer).Still one week later more fish are dying (6).The fish in there look gasping at the surface with a slight white film over them, this can especially be seen on the black fish.There are also a lot of rare newts in the pond.So we need help It's so sad to see, can anyone help please?
Boss - 29-Jun-14 @ 11:18 AM
@Sue. Have you tested the water quality? It seems as though the fish are looking for the freshest water (where the movement is). In the winter pond fish become slower and will often be seen 'lined up' facing the same direction in warm pockets of air...but can't see how that would apply at this time of year in the UK? Sorry not to be of more help.
Polyanthus - 9-Jun-14 @ 12:30 PM
We have a large pond and for the last 2weeks the fish have been behaving oddly. They are grouped together almost all of them are facing the same way (towards the waterfall) and they're just staying quite still. I thought perhaps that one fish was laying eggs but surely it wouldn't go on for 2 weeks? Any advice would be welcome!
Sue - 7-Jun-14 @ 12:42 PM
Hi We have koi carpgoldfish etc in our pond,its well established as its been built 12 years ago .we've notice alot of the fish especially the big ones seem to be going to the bottom of the pond and staying at the bottom also some of them are right at the top they also are not swimming about like usual also they are floating on the top. Any ideas or advice .Thankyou
finny - 17-Aug-12 @ 8:14 PM
Some of the gold fish in our pond are grouping on the surface and splashing around with each other, rubbing up against each other and the weeds, they seem to be swimming fin and dive away if you approach. there are no obvious signs of a problem so is this just natural behaviour - breeding/spawing etc? Thanks
Phil - 5-Aug-12 @ 10:22 AM
We have had our garden ponds for over 20 years but today our fish have started to die. Even when they are dead they look healthy, bright eyes good colour etc and. no visible signs of disease. Have you any advice please?
Lyndy - 2-Jun-12 @ 6:05 PM
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