Marine Fish Diseases – Identification & Treatment

Spotting a disease early and treating it correctly is the difference between saving a fish and losing the whole tank. This guide covers the most common diseases seen in marine aquariums, what they look like, how to tell them apart, and which treatments we sell. Every product mentioned links directly to our store.

All treatment should be carried out in a quarantine tank. Never dose a display aquarium containing corals, live rock, or invertebrates, as most marine fish medications will kill them. If you are unsure what you are dealing with, get in touch with our team before treating.

This guide is for general information only. Always read and follow the manufacturer's instructions on any medication.

Quick Reference

1. Marine Ich (White Spot Disease)

What causes it

Marine Ich is caused by the protozoan parasite Cryptocaryon irritans. It is the most common disease in marine aquariums. The parasite has a multi-stage life cycle: it feeds on the fish during the stage where white spots are visible, then drops off, forms a cyst on the substrate, divides into hundreds of new parasites, and releases them as free swimmers looking for a new host. At typical reef temperatures the full cycle takes anywhere from 3 to 28 days.

What it looks like

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Small white spots resembling grains of salt scattered across the body, fins, and gills. Each spot is roughly 0.5 to 1 mm, noticeably larger and rounder than the dusty coating seen with Marine Velvet. Spots are easiest to see under a bright torch held at a low angle. The fish may look otherwise normal in the early stages.

Symptoms to watch for

• White salt-grain spots on body, fins, and gills
• Flashing or scratching against rocks, sand, or décor
• Rapid or laboured gill movement
• Hiding more than usual
• Loss of appetite
• Spots may disappear then reappear. This is the life cycle, not recovery.

The visible stage is only part of the cycle

When spots disappear from the fish, the parasites have dropped off to reproduce on the substrate. They have not gone. A display tank can remain infected for months after visible symptoms disappear. Marine ich is effectively impossible to eradicate from a display containing live rock, coral, or invertebrates without removing all fish and running the tank fallow. The recommended fallow period is at least 76 days at 26°C to starve the parasite of a host.

Treatment (quarantine only)

• Aquasonic Formalin/Malachite Solution targets the free-swimming parasite stage. Remove carbon, increase aeration, and dose precisely as directed on the label.
• Tank Transfer Method (no medication): Moving the fish to a clean, sterilised tank every 72 hours over 12 days breaks the cycle before parasites can reinfect. Full schedule in our Marine Quarantine Guide.
• Aquasonic Para-Gone can be added alongside ich treatment to address flukes and worms at the same time, which is worth doing as part of a thorough quarantine.

Prevention

Quarantine every new fish for at least four weeks before adding to the display. Marine Ich is almost always introduced on an unquarantined fish. See our Marine Fish Quarantining at Home guide.

2. Marine Velvet (Amyloodinium)

What causes it

Marine Velvet is caused by Amyloodinium ocellatum, a parasitic dinoflagellate. It is significantly more dangerous than ich. It spreads and kills faster, can destroy gills before any external signs appear, and should be treated as an emergency. Bacterial infections frequently accompany velvet and can kill the fish even after the velvet itself is successfully treated.

What it looks like

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An extremely fine golden, rust, or grey dusty coating on the body and fins, often described as the fish being dusted with gold or cinnamon powder. The individual spots are far smaller than Marine Ich spots. Shining a torch at a low angle across the body brings out the characteristic sheen. In fast-moving cases, external signs may be minimal even as the gills are being severely damaged.

Symptoms to watch for

• Fine dusty or velvety golden or rust-coloured coating on body and fins
• Rapid, laboured gill movement, often appearing before visible spots
• Swimming directly into a powerhead or return flow (seeking oxygen)
• Flashing and scratching
• Lethargy and loss of appetite
• Sudden death with little visible warning in severe cases

Act immediately if velvet is suspected. There are no reef-safe velvet treatments. Fish must be moved to a quarantine tank straight away.

Treatment (quarantine only)

• Aquasonic Marine Oodonex is formulated specifically for marine velvet and related protozoan parasites in saltwater aquariums.
• Aquasonic Formalin/Malachite Solution is also effective against velvet. Remove carbon, increase aeration, and follow label dosing precisely.

Prevention

Quarantine all new fish before display introduction. Velvet can be present with no visible signs on a newly purchased fish.

3. Brooklynella (Clownfish Disease)

What causes it

Brooklynella is caused by the protozoan Brooklynella hostilis. It is most commonly associated with clownfish but can infect other marine species. Unlike ich, Brooklynella completes its entire life cycle on or in the host with no free-swimming stage in the water. It is one of the fastest-killing marine diseases; a fish can die within 30 hours of symptoms first appearing.

What it looks like

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A milky cloudiness or whitish film over the skin, often first visible near the gills. As the infection progresses the outer skin layer begins to visibly slough or peel. Some cases present with ich-like spots or resemble velvet. If a clownfish develops sudden symptoms, treat for Brooklynella and act immediately.

Symptoms to watch for

• Loss of appetite, often the very first sign
• Milky or cloudy appearance to the skin
• Skin visibly peeling or sloughing away
• Rapid, laboured gill movement
• Excess mucus visible on the body
• Lethargy, faded colour, and rapid deterioration

Copper has no effect on Brooklynella. Formalin-based treatment is required.

Treatment (quarantine only)

• Aquasonic Formalin/Malachite Solution contains formalin, which is the established treatment for Brooklynella. Move the fish to a bare quarantine tank immediately and treat as directed. Increase aeration throughout.

Prevention

Quarantine all new fish, especially clownfish, before display introduction. Act at the first sign of appetite loss in a clownfish. Speed matters with this disease.

4. Flukes (Gill and Skin Trematodes)

What causes them

Flukes are microscopic parasitic flatworms (monogenean trematodes). Gill flukes (Dactylogyrus sp.) infest the gills; skin flukes (Gyrodactylus sp.) infest the skin and fins. They are extremely common in newly imported fish and frequently present alongside other parasites. Individual flukes are not visible to the naked eye.

What they look like

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Flukes are not visible on the fish. Diagnosis is based on behaviour. A freshwater dip (placing a saltwater fish in dechlorinated freshwater for 3 to 5 minutes) can help confirm gill flukes, as osmotic shock causes them to detach and become opaque, and they can sometimes be seen in the dip water.

Symptoms to watch for

• Flashing and scratching against surfaces with no visible white spots
• Rapid or laboured gill movement
• Excess mucus on the skin, which may appear cloudy or slimy
• Clamped fins
• Redness around the gills
• Lethargy and loss of appetite

Formalin, malachite, and copper do not effectively treat flukes. A praziquantel-based treatment is required.

Treatment (quarantine)

• Aquasonic Para-Gone is a praziquantel-based treatment for flukes, worms, and internal and external parasites in both marine and freshwater fish. An initial dose plus a repeat treatment around one week later is typically required to address the full life cycle.

Prevention

Including a praziquantel treatment as part of standard quarantine for all new marine fish is strongly recommended, even when no symptoms are present. See our Marine Fish Quarantining at Home guide.

5. Uronema (Scuticociliatosis)

What causes it

Scuticociliatosis is the collective name for disease caused by histophagous (tissue-eating) ciliates of the subclass Scuticociliatia. In marine ornamental fish the species most often named is Uronema marinum, which was first described as a fish pathogen from specimens held at the New York Aquarium (Cheung et al., 1980).

These ciliates are opportunists rather than obligate parasites. They can live free in a marine system, and they turn on fish that are stressed, injured, or immune-compromised. Published reports link outbreaks to stress, handling, temperature swings, and changes in water quality, which is why the disease shows up so often in newly imported or recently shipped fish. Because the ciliate does not need a fish to survive, running a tank fallow is not a reliable way to clear it.

The parasite secretes proteases, including metalloproteases, which digest host tissue. That is why lesions deepen so quickly once they start.

What it looks like

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Open ulcerative lesions on the body surface and fins, often red or bloody-looking, and easily mistaken for a bacterial ulcer or a physical injury. Bulging eyes (exophthalmia) is a commonly reported sign, and in advanced cases the visceral cavity becomes swollen. The lesion typically starts as a small pale or reddened patch on the flank and erodes into a crater.

Which fish get it

A 2020 Brazilian study at an ornamental fish wholesaler documented Uronema spp. in ten reef species: blue-green and Vanderbilt's chromis, sea goldie (anthias), orchid dottyback, pennant coralfish, threespot and goldspotted angelfish, and threadfin, vagabond and bluecheek butterflyfish. The wholesaler reported a 20% mortality rate among fish showing these lesions. Chromis and other damselfish are the group most often affected in the aquarium trade.

Symptoms to watch for

• Red, open or crater-like sores on the flank, head, or fins
• Bulging eyes
• Rapid or laboured gill movement
• Loss of appetite and lethargy
• Swollen belly in advanced cases
• Rapid deterioration and death, often within days

It does not stay on the skin

This is the critical point that separates Uronema from an ordinary surface parasite. Histopathology in the published cases found the ciliates in the muscle, gills, gut and stomach, with associated damage in the liver, kidney and spleen. Other studies report invasion of the visceral cavity, pancreas, swim bladder and brain. By the time deep ulceration is visible the prognosis is poor.

Secondary bacterial infection is the norm, not the exception. In the Brazilian study, Shewanella putrefaciens was isolated from most lesions, along with Plesiomonas shigelloides and Vibrio alginolyticus.

Treatment (quarantine only)

• Aquasonic Formalin/Malachite Solution. Formalin is the established chemical approach to external ciliate infections. Move the fish to a bare quarantine tank, remove carbon, increase aeration, and follow label dosing precisely.
• Aquasonic Marine Vertonex to address the secondary bacterial infection that almost always accompanies the ulcers.

Treat early. Fish with deep, bleeding ulcers and internal involvement rarely recover, and euthanasia is often the kinder outcome. If you are unsure, get in touch with our team before treating. Browse all Fish Medication and Supplements.

Prevention

Quarantine every new fish, keep handling and transport stress to a minimum, and hold water quality and temperature steady. Because the ciliate is opportunistic, the single most effective control is not letting fish become stressed or injured in the first place. See our Marine Fish Quarantining at Home guide.

6. Black Spot Disease (Turbellarian Worms)

What causes it

Black spot disease in tropical marine fish is caused by parasitic flatworms of the class Turbellaria. Two genera are documented as causing it: Paravortex and Piscinquilinus. Piscinquilinus is the current name for the genus long known in the hobby as Ichthyophaga; it was renamed in 2005 because the original name was already in use for a bird.

The black spot is not the parasite. The worm burrows into the skin, and the fish walls it off in a cyst. The cyst wall contains melanocytes, so what you are seeing is the fish's own pigment response. This matters practically: a spot can persist after the worm inside it has died, and some fish keep dark scarring after a successful treatment.

What it looks like

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Small dark spots scattered over the skin, fins and sometimes gills. Paravortex spots are small, often well under a millimetre, and look like grains of pepper. Piscinquilinus produces larger raised swellings; in the published New Caledonia case these were 2 to 4 mm across with no visible opening, each containing a single whitish worm 2 to 4 mm long.

Which fish get it

Tangs and surgeonfish are by far the most commonly affected group in aquariums. A six-year survey of more than 2,500 wild reef fish in New Caledonia also recorded turbellarian infections on butterflyfish, wrasse, threadfin bream, damselfish and rockcod. Paravortex species infest a wide range of fish families, whereas Piscinquilinus appears more host-specific: in the documented aquarium case the infection passed to other unicornfish sharing the tank but not to fish of other families.

Worth knowing: infection is rare in wild reef fish. That New Caledonia survey found only seven cases in over 2,500 fish, a prevalence of 0.3%.

Symptoms to watch for

• Small dark spots on the body, fins and gills
• Flashing and rubbing against rock or substrate
• Loss of colour and appetite
• Lethargy
• Spots increasing in number over days rather than staying static

Life cycle

Paravortex and Piscinquilinus are both understood to have a single-host life cycle, with no intermediate host required. Paravortex are viviparous and release many juveniles, and one species has been shown to complete its cycle in about ten days, which is why an infestation can spread quickly in a closed system. Piscinquilinus appears slower: in the New Caledonia case the worms remained on the fish for at least 30 days, and the reproductive organs were undeveloped, suggesting the worm only matures after it leaves the host.

Not every black spot is this

Black spots in fish are also caused by encysted digenean flukes (metacercariae), which are a completely different parasite. Under a microscope the two are easy to separate once released from the cyst: turbellarians have cilia and glide, while metacercariae have no cilia and move by peristalsis. Clownfish can also develop dark markings that are not parasitic at all.

Treatment (quarantine)

The evidence here is worth stating plainly, because there is a lot of contradictory advice in circulation. In the published aquarium case, seventeen days of copper sulphate at 15 mg/L combined with reduced salinity failed to cure the infected fish and did not stop the infection spreading to tankmates. The infection was only cleared after two praziquantel treatments, with the spots fading progressively over the following weeks.

• Aquasonic Para-Gone is praziquantel-based. A repeat treatment is needed, as a single dose is unlikely to clear the infestation.

Two honest caveats. First, in the published study the praziquantel was given by injection under professional care, not as a bath, so the same result cannot be assumed from immersion dosing alone. Second, the cyst wall the fish builds around the worm may slow the drug reaching it, which is the likely reason two treatments were needed. Increase aeration during treatment and monitor water quality with a test kit. Browse all Fish Medication and Supplements.

Prevention

Quarantine all new fish, and inspect tangs and surgeonfish carefully under a bright light before purchase and again before they go into the display.

7. Bacterial Infections and Fin Rot

What causes them

Bacterial infections in marine fish are caused by opportunistic bacteria including Vibrio, Aeromonas, and Pseudomonas species. They rarely take hold in a healthy, unstressed fish and typically follow injury, parasitic infection, shipping stress, or persistent poor water quality. Fin rot is one of the most common presentations.

What it looks like

Frayed, ragged, or receding fin edges, often with a white or reddish margin at the deteriorating edge. More advanced infections present as open ulcers or red lesions on the body, redness at the fin base, or cloudy eyes. Bacterial septicaemia (a systemic infection) shows as red streaks through the body, swelling, and rapid decline.

Symptoms to watch for

• Fin edges appearing ragged, frayed, or receding
• Redness at the base of fins
• Open wounds, ulcers, or sores on the body
• Cloudy or sunken eyes
• Red streaks across the body (septicaemia, serious)
• Lethargy and loss of appetite

Always check water quality first. Test with a test kit before medicating. Bacterial infections rarely take hold in a well-maintained system.

If the ulcer is deep, crater-like and spreading fast, particularly on a chromis, damsel or anthias, consider Uronema (section 5) rather than a straightforward bacterial ulcer. The two look similar and are frequently found together.

Treatment (quarantine)

• Aquasonic Marine Vertonex treats bacterial infections including fin rot and ulcers in marine fish. Treat in quarantine, remove carbon, and follow label dosing.

Prevention

Stable water quality, appropriate stocking, and avoiding physical injury from aggressive tankmates. Test parameters regularly with a test kit.

8. HLLE (Head and Lateral Line Erosion)

What causes it

HLLE is a chronic condition most commonly seen in tangs, surgeonfish, and angelfish. It is linked to nutritional deficiencies (particularly vitamins C and A), the use of activated carbon, poor water quality, and chronic stress. It is not infectious or contagious.

What it looks like

Pale, grey, or white pitting and erosion of tissue on the forehead and along the lateral line (the sensory line running along the fish's side). Early stages appear as faded patches or small pits. Advanced cases show significant erosion. The fish often continues eating and behaving normally.

Symptoms to watch for

• Pale or discoloured patches on the forehead and around the eyes
• Pitting or cratering of tissue on the head
• Progressive erosion along the lateral line
• Fish otherwise eating and behaving normally in early stages

Management

There is no specific medication for HLLE. Management focuses on addressing the likely causes:

• Remove or replace activated carbon
• Improve diet with variety, algae, and vitamin-enriched foods
• Improve and maintain water quality with regular testing using a test kit
• Reduce stress through appropriate tank size, compatible tankmates, and good flow

Partial tissue recovery is possible over time with consistent improvements, though full recovery varies between individual fish.

Shop Treatment Products

• Aquasonic Marine Oodonex - Marine Velvet
• Aquasonic Formalin/Malachite Solution - Ich, Velvet, Brooklynella, Uronema
• Aquasonic Para-Gone - Flukes, Worms, Black Spot (Turbellarians)
• Aquasonic Marine Vertonex - Bacterial Infections, Fin Rot, Secondary Infection
• Test Kits and Monitoring
• All Fish Medication and Supplements

Sources

The Uronema and black spot sections on this page are drawn from the following published veterinary and parasitology literature.

• Cardoso PHM, Moreno LZ, Moreno AM, Balian SC, Oliveira CH, Soares HS, Martins ML, Rodrigues MV (2020). Scuticociliatosis caused by Uronema sp. in ten different ornamental aquarium reef fish in Brazil. Revista Brasileira de Parasitologia Veterinária 29(1): e018319. doi:10.1590/S1984-29612019104
• Cardoso PHM, Balian SC, Matushima ER, Pádua SB, Martins ML (2017). First report of scuticociliatosis caused by Uronema sp. in ornamental reef fish imported into Brazil. Revista Brasileira de Parasitologia Veterinária 26(4): 491-495. doi:10.1590/s1984-29612017031
• Cheung PJ, Nigrelli RF, Ruggieri GD (1980). Studies on the morphology of Uronema marinum Dujardin (Ciliatea: Uronematidae) with a description of the histopathology of the infection in marine fishes. Journal of Fish Diseases 3(4): 295-303.
• Justine JL, Leblanc P, Keller F, Lester RJG (2009). Turbellarian black spot disease in bluespine unicornfish Naso unicornis in New Caledonia, caused by the parasitic turbellarian Piscinquilinus sp. Diseases of Aquatic Organisms 85: 245-249. doi:10.3354/dao02082
• Cannon LRG, Lester RJG (1988). Two turbellarians parasitic in fish. Diseases of Aquatic Organisms 5: 15-22.
• Kent ML, Olson AC (1986). Interrelationships of a parasitic turbellarian (Paravortex sp.) (Graffillidae, Rhabdocoela) and its marine fish hosts. Fish Pathology 21: 65-72.
• Sluys R, Kawakatsu M (2005). A replacement name for Ichthyophaga Syromiatnikova, 1949 (Platyhelminthes: Prolecithophora), with a nomenclatural analysis of its avian senior homonym. Species Diversity 10: 63-68.

Disclaimer

This guide is provided for general educational purposes only. It is not a substitute for professional veterinary or aquatic health advice, and we cannot guarantee outcomes. Aquarium keeping involves inherent variables including species sensitivity, water quality, transport stress, and system design, all of which affect results. Always read and follow the manufacturer's instructions on any product you use. If you are unsure about a diagnosis or treatment, please get in touch with our team before proceeding.