How to Identify and Treat Common Pond Diseases


What types of disease can affect the everyday garden pond? We take a look at the common maladies that can make outdoor aquatics a nightmare.

The ornamental pond fish in the UK often gets a rough deal. Harassed by herons, baked in the sun, frozen in the winter, often overcrowded or under-filtered—it’s a miracle that pond diseases aren’t more common.

Common causes of pond disease: What are they?

Broadly speaking, there are a few scenarios that can result in a breakout of pond fish disease. Here are some of the most common...

1. Temperature Increase in Spring

Pond disease is often caused by the rising temperature of springtime. As water slowly warms after winter, any pathogens lingering in the pond—parasites, fungus, bacteria and viruses—become active at a faster rate than the fishes’ immune systems.

2. Adding New Fish to the Pond

The second is the addition of new fish. Any introduction of livestock can carry pathogens to which your existing fish are ‘naïve’ and have no immunity to.

3. Stress

Stress from some underlying factor within the pond itself. Usually this will be water quality based, such as a build-up of ammonia or nitrite from a struggling filter system, though old ponds may also suffer with water chemistry issues such as a drop in pH and hardness.

4. Physical Harm: Wounds and infection

The fourth reason is stress and physical harm from something outside of the pond. Herons in particular are notorious not just for eating fish, but also wounding those that they don’t catch outright. This can have a knock-on affect on the pond's chemistry.

Any of the above can lead to a wide range of disease pathogens manifesting in the pond.

Here are some of the most common, and some not so common illnesses to look for.

Gasping and gill issues

Fish gills are delicate and sensitive organs, easily affected by a range of problems. The first sign that anything is wrong with gills is a fish breathing erratically, or gasping at the pond’s surface. It also requires immediate intervention. 

Low oxygen can cause gasping and heavy breathing, and exacerbate a pathogenic gill issue, so first make sure that the pond has sufficient turnover and oxygenation. Turn waterfalls on (especially at night), make sure that flow from pumps and filters is breaking the water’s surface, and consider using an airstone connected to an airpump to increase movement.

High temperature can cause gasping by lowering oxygen content of water, so check the temperature. If it’s above 25°C, that’s a likely cause.

Ammonia and nitrite can cause gill problems. Look for gills that appear burnt and red for ammonia poisoning, and gills that appear brown for nitrite poisoning. Test the water and check that everything is as it should be. 

Bacterial gill disease is a pernicious illness that causes gill tissue to turn necrotic (rotten). In the event of a gill disease outbreak, consult a vet as powerful prescription medication will likely be required.

Gill flukes (Dactylogyrus) cause similar symptoms to skin flukes (see below) and require a microscope to diagnose. If gill flukes are diagnosed, then you can treat with Colombo Lernex Pro.

Sliminess of the skin

Skin slime may have one or more of several causes, usually parasitic or water chemistry/quality based.

Spotting sliminess is easy. The fish will have a milky or slightly grey sheen across them, either in patches or covering the whole body. In extremes, the slime may even slough off of the fish in sheets or strands. 

Poor water quality can cause irritation to fish, which in turn causes a build-up of protective mucus. Start by testing for ammonia, nitrite, pH and carbonate hardness. If all of these are as they should be (zero ammonia, zero nitrite, pH between 7.0-8.0, KH 5-8°dKH) then it’s time to look for parasites.

Gyrodactylus, a type of skin fluke, will cause increased slime, but this will require a microscope to correctly diagnose. Fish carrying these flukes will often scratch themselves against solid objects in the pond, as well as flick, jump, and appear agitated. Other parasites that can cause sliminess include Ichthyobodo, Chilodonella, and Trichodina, and present the same symptoms in fish. 

All three of these pathogens can exist at a low level on a fish, living ‘under the radar’ and not causing symptoms. Once the fish’s immune system becomes compromised, they multiply until they become a burden.

Treatment involves a course of anti-parasite medication, such as Cloverleaf Absolute Parasite+, Blagdon Anti-Parasite, Colombo Alparex, NT Labs Anti-Parasite & Fungus, or Tetra Pond Medifin.

Eye problems

Cloudy eyes can be caused by wounds, parasites, poor water quality, and poor diet. In the first instance, test your water and then check that your foods are in date and haven’t been opened for more than a few months (pond food can spoil quickly once opened).

In the case of a physical wound, monitor the situation and have a broad range antibacterial treatment on standby such as NT Labs Anti-Ulcer, Finrot & Flukes, in case the fish deteriorates.

Bulging eyes can be an indication of popeye (Exophthalmus) where bacteria have gotten behind the eye, or they can be an early sign of dropsy. Treat with an antibacterial medication like Blagdon Anti Ulcer, but be prepared to isolate the fish if the scales start to stick out and the whole fish bulges.

Open wounds and ulcers

Physical injuries to a fish are usually obvious, with cuts plainly visible. These can be from knocking against something in the pond, or an animal attack such as a heron or cat.

In the biological soup that is pond water, infections can set in quickly, so when a lesion is spotted, it should be treated directly with an NT Labs Paramedic Kit. This will involve removing the fish from the pond, sedating it, cleaning the wound and then sealing it—the kit contains all you need to do this.

Ulcers start out looking like red sores anywhere on the fish, developing into a hole with a white edge. Ulcers can quickly be fatal if not treated. In the first instance, cleaning with the NT Labs Paramedic Kit will help, especially if followed up by a course of NT Labs Anti-Ulcer, Finrot & Flukes treatment in the pond. Extremely aggressive ulcers that don’t respond to treatment early on will need prescription medicines from a vet. 

Frayed and damaged fins

Split fins are often the result of rough handling, and as long as the conditions in the pond are sanitary they will eventually heal over and repair themselves. Have a bottle of broad antibacterial medication on standby if things deteriorate, but don’t panic if the area looks clean.

If the fins rays are intact but the delicate skin between them is rotting away (often with a white tinge or edging) then you are looking at a far more dangerous incident of finrot, caused by various pathogenic bacteria. This will need immediate treating, as if the bacteria reach the body the disease will become systemic, infecting the entire fish and killing it quickly.

In the event of finrot, begin by looking at the conditions in the pond. This infection often goes hand in hand with poor pond hygiene, so a partial waterchange and removal of detritus from the base of the pond and from inside filters is a must.

To treat, there are many generic ‘anti finrot’ medications on the market, and these are good at treating the early stages of finrot. For a severe infection (especially where most of the fin has eroded away) consider treating solely with acriflavine.

Obvious parasites

A couple of larger parasites that can become visible on a fish are fish lice (Argulus) and anchor worms (Lernaea). Unless introduced on new fish, both are extremely unlikely to appear in a garden pond.

Fish lice look like small olive-green scales that cling to the fish’s body. Typically, they move about, including from fish to fish. A heavy infestation will weaken a fish to the point of death, with the resulting bite marks left on fish can become infected, leading to bacterial infections and ulcers. If you see fish lice, and it is possible to catch the fish easily, they can be carefully removed with a pair of tweezers. The pond will still need treating, however. 

Anchor worms, despite the name, are crustaceans with a hooked head that is buried in the flesh of a fish. The point of entry is usually red and inflamed, and the parasite is visible, often with two white ‘tails’ which are the egg sacs of females. Never try to remove an anchor worm physically, as the head will either cause considerable damage coming out, or may break off and remain in the fish, leading to a deeper infection.

An infection of either will cause fish to appear irritated, scratching and flicking against objects in the pond. Secondary lesions may also be observed from parasite attacks.

In a case of fish lice or anchor worms, a treatment of emamectin, such as in Vetark Lice-Solve should be used to kill off both adult and larval parasites.

Fluffy growths

Distinct fluffy patches growing on a fish are indicative of a fungal infection. These white tufts can appear at any part of the fish, though frequently they form at the site of an injury. In extreme cases, fungal growth can appear green as the fungal strands become contaminated with algae growing on them.

Fungus is almost always associated with poor hygiene and water conditions, and these will need to be addressed before any treatment can take place. Perform some water changes and remove detritus from the pond base and filters.

While fungus visually looks limited to the surface of the fish, hyphae are boring into the fish’s tissues, destroying them and providing nutrition for yet more fungi. Never ignore a fungus tuft.

Treatment is straightforward and can be carried out with any broad-range anti-fungal medication such as Blagdon Anti Fungus & Bacteria. Improvement of water conditions is vital for treatment to work though.

Waxy white lumps

Small lumps that look like the fish has been dripped with candle wax is an indication of carp pox, and appears especially common in strains of koi.

Carp pox is a viral infection that cannot be treated. Symptoms usually appear throughout winter and disappear on their own at the arrival of spring. The only time to worry about them is if they also become infected with fungus or bacteria, in which case you should treat for whatever the opportunistic pathogen is.

Bulging fish

A fish that looks distended and bloated may be suffering from one of a few conditions.

In the first instance, it’s important to differentiate between an illness and an adult female fish filling with eggs. A fish carrying eggs will look larger than usual, but will not have buoyancy issues, will not have bulging eyes, will not have scales sticking on end, and will be behaving normally. It’s also normally the case that she will be pursued by all and nay males in the pond.

If the fish is bloated to the extent that the scales are protruding (think of an open pine cone) then the fish is suffering from one of the many illnesses under the umbrella term of dropsy. Dropsy describes symptoms rather than the illness itself, but in all cases the fish has lost its ability to regulate its fluids properly, leading to an accumulation within the body.

Dropsy may be bacterial, it may be from a physical injury, it may be age related, it may be diet related, and it may be genetic. It is also incredibly hard to treat (in part as diagnosing the cause is difficult. In the case of bacteria dropsy, off-the-shelf antibacterial medications may help, but more often a vet will be required to prescribe antibiotics. The outlook of a dropsy diagnosis is bleak and the fish should be isolated in a different body of water to the other pond fish. 

Ingested gas from feeding at the surface will also cause bloating specifically in the gut region, and will also cause swimming and buoyancy issues. At its most extreme, the fish may be belly-up and floating at the pond’s surface. If this is a recurrent problem, consider using a sinking pellet food instead of floating foods. 


In a pond sited in direct sunlight, it is more than possible for fish to suffer from sunburn, expressed as a red rash across white parts of the body. Sunburn can compromise the fish’s immune system by damaging the mucous layer, leaving the fish open to subsequent bacterial or fungal infections.

While there’s no medication directly for sunburn in fish, symptoms should be addressed by providing the fish with ample cover in the pond. This may involve the addition of floating plants, tall marginal plants, and wide-leafed waterlilies, or even adding some cover in the form of basic caves for fish to retreat into.

Small white spots

Whitespot, caused by the protozoan parasite Ichthyophthirius, is one of the classic fish diseases, and outbreaks are largely the result of new additions to a pond. Symptoms may not be apparent until you see the fish quite close up, but early warning signs include flicking, scratching, gasping at the surface, and increased slime production. An infected fish will look at though it has been flecked with tiny white spots of paint, that affect any part of the body.

Whitespot is highly infectious and can spread through a pond rapidly, so treatment at the first sign is essential.

Treatment is simple, and there are many excellent anti-whitespot medications on the market, such as Colombo FMC-50. Just be sure to follow a complete course of whichever medication you use, even if symptoms disappear within a day or two— Ichthyophthirius has a complicated lifecycle with multiple stages, and only the free-swimming stage is usually susceptible to treatment.