Ich (Ichthyophthirius multifiliis): The White Spot Disease in Freshwater Fish

“Ich,” also known as “ick” or “white spot disease,” is one of the most common and potentially fatal parasitic infections in freshwater fish. Caused by the ciliated protozoan parasite Ichthyophthirius multifiliis, this disease affects both aquarium fish and commercial species in aquaculture, leading to significant losses if not controlled promptly.

If you have ever wondered about the white spots on your fish, what this disease looks like, or how to cure it, this comprehensive and updated guide will provide all the key information you need. This article will delve into what white spot disease is, its visible and behavioral symptoms, underlying causes, the parasite’s crucial life cycle, the most effective treatments (from commercial options to advanced methods), and the best prevention strategies. Additionally, we will explore whether this disease is dangerous to humans and how you can prevent its spread in your aquarium or fish farm.

Key takeaways

• White spot disease is caused by the parasite Ichthyophthirius multifiliis and is treatable if detected early.
• Treatment is only effective during the parasite’s “theront” stage when it is free-swimming in the water.
• Raising the temperature to 28-30°C (82-86°F) is a critical step in almost all treatments to accelerate the parasite’s life cycle.
• Prevention, through a strict 4–6 week quarantine for new fish, is the most effective way to avoid outbreaks.
• Commercial treatments are the safest option for most, while the use of pure chemicals like malachite green or formalin requires experience and caution.

What is Ich, white spot disease, or Ichthyophthiriasis?

White Spot Disease is caused by the parasite scientifically known as Ichthyophthirius multifiliis and commonly referred to as “Ich” or “Ick.” It is an obligate parasite, meaning it cannot survive without a living fish host (Francis et al., 2023). The parasite is highly contagious and can quickly spread throughout an aquarium or pond, leading to significant mortality if left untreated.

Ichthyophthiriasis infects a wide range of freshwater fish species in aquaculture, aquariums, and ornamental fish. The disease is prevalent year-round, thriving in a broad temperature range (11°C to 32°C), suggesting the existence of both tropical and temperate strains of I. multifiliis (Yang et al., 2023).

The name “White Spot Disease” comes from the visible white cysts that form on the skin, fins, and gills of infected fish as the parasite burrows into the tissue. However, not all “white spots” on fish indicate Ich. Other conditions, such as lymphocystis or fungal infections, can cause similar symptoms. Proper diagnosis is essential for effective treatment.

It is also important to note that there is a marine version of White Spot Disease, known as “Marine Ich,” which affects saltwater fish and is caused by the parasite Cryptocaryon irritans.

Key characteristics of Ichthyophthirius multifiliis

• Parasitic nature: Ichthyophthirius multifiliis is a ciliated protozoan parasite.
• Host specificity: Primarily infects freshwater fish.
• Life cycle: The parasite has a complex life cycle, including both free-swimming and host-attached stages.

Taxonomy of Ich

• Domain: Eukaryota
• Clade: Diaphoretickes
• Clade: SAR
• Clade: Alveolata
• Phylum: Ciliophora
• Class: Oligohymenophorea
• Order: Hymenostomatida
• Family: Ichthyophthiriidae
• Genus: Ichthyophthirius
• Species: Ichthyophthirius multifiliis
• Common name in Spanish: Ich, enfermedad del punto blanco
• Common name in English: Ich, Ick, White Spot Disease

What does Ich look like in fish?

One of the most common questions among aquarists is: “What does Ich look like in fish?” The symptoms of White Spot Disease are relatively easy to identify, making it one of the most recognizable fish diseases.

Symptoms of white spot disease in fish

Identifying white spot disease in its early stages is crucial for successful treatment. Pay close attention to two types of symptoms: physical and behavioral.

Physical symptoms

• White Spots: The most evident sign is the appearance of small white spots or cysts on the fish’s body, fins, and gills, resembling grains of salt or sugar. These are the individual parasites (trophonts) embedded under the fish’s skin.
• Excess mucus: Fish may develop a grayish mucus layer on their skin as a response to the irritation.
• Fin and skin damage: In advanced cases, the fins may appear frayed, and lesions or ulcers can develop on the skin.
• Cloudy eyes: In severe instances, the fish’s eyes may become cloudy or opaque.

Behavioral symptoms

• Rubbing (Flashing): Fish may erratically and nervously rub against objects in the aquarium, such as rocks, ornaments, or the substrate, in an attempt to relieve the irritation caused by the parasite.
• Clamped fins: They often hold their fins close to their body, a common sign of stress and illness.
• Lethargy and lack of appetite: Infected fish frequently lose interest in food and become apathetic, often staying at the bottom of the tank or hiding.
• Labored breathing: If the parasites infest the gills, fish will have difficulty breathing. You may observe them gasping for air at the water’s surface.

If you notice any of these symptoms, it is essential to act quickly to prevent the spread of the disease. However, it is important to confirm the diagnosis through a microscopic examination of skin and gill scrapes. It is crucial to distinguish between the immature forms (tomites) of Ichthyophthirius and other protozoa such as Tetrahymina, as the latter generally does not require treatment (Francis et al., 2023).

Recently, Li et al., (2023) presented an artificial intelligence (AI)-based method for the rapid detection of fish parasites, specifically I. multifiliis, Gyrodactylus kobayashii, and Argulus japonicus. Meanwhile, Bonnichsen et al., (2025) developed a real-time AI-based video system for detecting “Ich” in rainbow trout.

The life cycle of Ichthyophthirius multifiliis

Understanding the life cycle of Ichthyophthirius multifiliis is essential for effective treatment. The parasite goes through several stages, each with unique characteristics.

Stages of the Ick life cycle

Jørgensen (2017) reported that the three stages of the Ich disease life cycle are:

• Theront Stage (Infectious Stage): The cyst ruptures and releases new parasites (theronts), which swim freely in the water searching for a new host fish. The theront invades the fish’s skin and gills, penetrates the epidermis, and settles on the basal lamina. This is the only stage where the parasite is vulnerable to chemical treatments and medications. If they do not find a host within 24-48 hours, they die.
• Trophont Stage (Feeding Stage): This is the visible white spot phase on the fish. The parasite feeds on the host’s tissues, protected under the mucus layer and epidermis, until it reaches a size of 0.5 to 1.0 mm and becomes macroscopically visible as a white spot. The mature trophont then exits the fish and transforms into a tomont. At this stage, it is resistant to medication.
• Tomont Stage: After feeding, the parasite leaves the host and forms a cyst on surfaces like the tank walls or substrate. Inside this cyst, it divides into hundreds of daughter cells.

This life cycle can last from a few days to several weeks, depending on water temperature and other environmental factors. Treatment should focus on the tomite stage, as this is when the parasite is most vulnerable.

How does Ich spread?

White spot disease is highly contagious and can spread through:

• Introduction of infected fish: Adding new fish to an aquarium without proper quarantine can introduce the parasite.
• Contaminated equipment: Sharing nets, siphons, or other equipment between tanks can spread the parasite.

Risk factors

• Stress: Poor water quality, overcrowding, or sudden temperature changes can weaken the fish’s immune system, making them more susceptible to infection.
• High fish density: Overcrowded tanks provide an ideal environment for the parasite to spread.
• Poor water quality: The accumulation of ammonia, nitrite, and nitrate can stress fish and promote disease. In this regard, Guang-Ran et al., (2024) found that in ponds with Ichthyophthiriasis outbreaks, nitrate concentrations were negatively correlated with the abundance of I. multifiliis genes.
• Temperature fluctuations: Ich thrives in cooler waters, but rapid temperature changes can also stress fish.

A guide to treating white spot disease

The treatment for Ich must be administered quickly and encompass the entire aquarium, not just the visibly affected fish, as the parasite is free-swimming in the water.

Crucial steps before starting any treatment

1. Raise the temperature (Thermotherapy): Gradually increase the aquarium temperature to 28-30°C (82-86°F). This accelerates the parasite’s life cycle, forcing trophonts to detach and tomonts to release theronts faster, thereby exposing them to medication. Notably, Jørgensen (2017) reported that a temperature of 32°C (89.6°F) can destroy the parasite, although its application in large-scale aquaculture is limited.
2. Remove activated carbon: If your filter contains activated carbon or other chemical filtration media, remove them. These materials will absorb the medication from the water, nullifying its effect.
3. Increase aeration: Warmer water holds less oxygen. Add an air stone or increase surface agitation to ensure your fish can breathe comfortably.

Option 1: Commercial treatments (Recommended for beginners)

For most aquarists, using commercial medications is the safest and most effective option, as these products are formulated with precise dosages. Look for those containing active ingredients like malachite green, formalin, or a combination of both.

• Popular medications: Some well-regarded products on the market include Seachem ParaGuard, API Super Ick Cure, Kordon Rid-Ich Plus, and Ich-X.
• Instructions: Always follow the manufacturer’s instructions to the letter. Treatment typically lasts 10 to 14 days to ensure all parasite stages are eradicated. Continue medicating for at least 3-5 days after the last white spot has disappeared.

Option 2: Thermotherapy and aquarium salt

This method is a gentler alternative, ideal for tanks with chemically-sensitive fish (such as scaleless fish, tetras, or corydoras) or invertebrates.

1. Raise the temperature as described above.
2. Add Aquarium Salt (Non-Iodized): Slowly dissolve 1-3 grams of aquarium salt for every liter of water (approximately 1-2 tablespoons per 5 gallons). The salt helps damage the parasites through osmosis and improves the fish’s gill function.
3. Perform Water Changes: Conduct partial water changes every 2-3 days, replenishing the salt amount with the new water you add.

Option 3: Advanced chemical treatments (For expert use)

Warning: This method requires precision and is potentially dangerous to both the fish and the user if not performed correctly. It is recommended for advanced aquarists or aquaculture professionals only.

• Malachite Green: A stock solution can be prepared and dosed carefully. It is effective but toxic in overdose and can stain silicone seals and aquarium decorations. This chemical acts against the parasite’s free-swimming stages and those within the fish’s epidermis. However, carcinogenic and teratogenic effects of malachite green have been reported, leading to its ban for use in food fish in several countries.
• Formalin (37% Formaldehyde Solution): This is highly effective, especially in combination with malachite green. It is a potent chemical that must be handled with gloves and in a well-ventilated area. A typical dose is approximately 1 ml per 40 liters of water, but this can vary. It is lethal to parasites at concentrations of 50-100 mg/L for 30-60 minutes and can be tolerated by certain fish species like rainbow trout. However, it has been classified as a carcinogen.
• Copper sulfate: Effective in killing theronts at concentrations of 1 mg/L for 13 minutes, though its toxicity depends on water parameters. Copper sulfate prevents transmission but can be toxic to fish in low-alkalinity waters and is not approved for food fish.
• Chlorine dioxide: In vitro studies demonstrated that 50 mg/L of chlorine dioxide for 1 hour killed theronts, and in vivo, a dose of 25 mg/L for 48 hours reduced parasite load by more than 50% in silver catfish.
• Compounds releasing hydrogen peroxide: Hydrogen peroxide is lethal to parasites at concentrations tolerable to fish. The median lethal concentration (LC50) at 4 hours of peracetic acid (PAA), which decomposes into hydrogen peroxide and acetic acid, against I. multifiliis theronts was 1.076 ppm for tilapia treatment (Abu-Elala et al., 2021).

Francis et al. (2023) recommend that the choice of chemical depends on water quality conditions, the fish species being treated, and the type of system in which the fish are kept.

Drugs

• Toltrazuril: An antiparasitic drug used in other animals that, under the “cascade” principle, can be used in fish if no medications are available for that species. It was found to protect against infection but does not affect already established parasites.

• Plant Extracts: Several extracts have been investigated for their antiparasitic effects.

• Essential oils of Melaleuca alternifolia, Lavandula angustifolia, and Mentha piperita: Showed in vitro toxicity against trophonts. M. alternifolia oil also showed in vivo potential in pacu (Piaractus mesopotamicus); Valladão et al., (2016) reported that 50 μL L⁻¹ had an efficacy of nearly 100% in eliminating parasites from fish skin and gills.

• Extracts of Cynanchum atratum, Zingiber officinale, and Cynanchum paniculatum: Effective in vitro against different parasite stages, although their application through feed had a limited effect in herbivorous carp. Cynatratoside-C, isolated from C. atratum, showed high efficacy in vitro and in vivo in approximately 3 hours at a low concentration of 0.25 mg/L against theronts in herbivorous carp, with low toxicity to the fish (Jørgensen, 2017).

• Gracillin and zingibernsis from Costus speciosus: Effective in vitro and in vivo against Ich in herbivorous carp.

• Extracts from the root bark of Morus alba (white mulberry): Showed in vitro efficiency. Kuwanones G and O, flavonoids from this plant, were lethal to theronts and reduced their infectivity.

• Chelerythrine and chloroxylonine extracts from Toddalia asiatica: Effective in vitro and in vivo in goldfish.

• Pentagalloylglucose from Galla chinensis: Lethal to Ich in vitro and effective in treating catfish and preventing infections in non-infected fish.
• Procyanidins: Procyanidins are compounds found in grape seeds, apples, and cocoa. Yang et al. (2025) determined that the median effective concentrations (EC50) were 0.10 mg/L for theronts (the infectious stage), 2.99 mg/L for protomonts, and 4.65 mg/L for tomonts (the reproductive stage). Furthermore, they reported that in experiments with goldfish, an immersion treatment in an 8.0 mg/L solution of procyanidins for 17 days completely cured ichthyophthiriasis.

• Psoralidin from Psoralea corylifolia: Highly effective in vitro against theronts and capable of disrupting parasite reproduction.

Actinobacteria

• Amphotericin B from Streptomyces sp. strain HL-2-14: Significantly reduced Ich infectivity and intensity in herbivorous carp in vitro and in vivo, although mortality was observed at higher concentrations.

• Nystatin from Streptomyces griseus SDX-4: Effective in vitro against theronts and encysted tomonts and reduced parasite mortality and reproduction in goldfish in vivo. It is important to consider that many of these compounds have antibiotic properties, making them less desirable due to the risk of generating antibiotic-resistant microorganisms.

Biological control strategies

• The use of the leopard pleco (Glyptoperichthys gibbiceps) in warm water systems significantly reduced I. multifiliis infections, as these fish feed on biofilms where the parasite settles to encyst.

• Copepods and other filter-feeding microorganisms can ingest tomonts and theronts, reducing infection pressure in the system. Wang et al., (2025) identified six copepod species as predators of I. multifiliis theronts, all belonging to the order Cyclopoida; however, Cyclops vicinus was the dominant predator, representing 82.72% of the total predators.

Physical barriers

Prakash et al. (2026) developed an “affinity silk” filter as an effective tool to prevent ichthyophthiriasis (white spot disease) by physically removing Ichthyophthirius multifiliis parasites from aquarium water. The silk is an engineered material that fuses a silk protein (FibL) with an antibody fragment (scFv) that specifically binds to the I. multifiliis parasite.

Prevention of white spot disease in fish

To prevent the occurrence of white spot disease in fish, follow these tips:

• Quarantine new fish before introducing them into the main aquarium.
• Maintain good water quality and avoid sudden temperature changes.
• Reduce stress in fish by providing hiding spots and a balanced diet.
• Disinfect decorations and plants before placing them in the aquarium.

• Boosting fish immunity: Jørgensen (2017) reported that immune response studies against I. multifiliis have been conducted mainly in rainbow trout, channel catfish, and common carp. Meanwhile, Buchmann (2020) stated that protective immunity development in fish against Ichthyophthirius multifiliis is based on a combination of strong innate and adaptive immune responses, where specific immunoglobulins, such as IgT, play a crucial role.

Is Ich dangerous to humans?

A common question among aquarists is: Is Ich dangerous to humans? The answer is no. Ichthyophthirius multifiliis is a fish-exclusive parasite and cannot infect humans. However, it is always advisable to wash your hands after handling sick fish or aquarium water.

Conclusion

White spot disease is one of the most feared diseases among aquarists and fish breeders. Knowing its symptoms, life cycle, and treatment methods will allow you to act quickly to save your fish.

Ich is an infection caused by I. multifiliis that, if left untreated, can be fatal. Maintaining good aquarium hygiene and following preventive practices will significantly reduce the risk of infection.

Have you experienced white spot disease in your aquarium? Share your experience in the comments and help other aquarists keep their fish healthy.

Frequently Asked Questions About White Spot Disease (Ichthyophthirius multifiliis)

To further assist aquarists and fish enthusiasts, here are answers to some frequently asked questions about Ich disease:

References

Abu-Elala, N. M., Attia, M. M., Abd-Elsalam, R. M., Gamal, A., & Younis, N. A. (2021). Peracetic acid treatment of Ichthyophthirius multifiliis (Ciliophora: Ichthyophthiriidae) and Trichodina spp. Reduces the infection by Aeromonas hydrophila and improves survival in Nile tilapia (Oreochromis niloticus). Aquaculture, 538, 736591. https://doi.org/10.1016/j.aquaculture.2021.736591

Bonnichsen, R., Brink Nielsen, G. G., Dam, J. S., Schrøder-Petersen, D., & Buchmann, K. (2025). AI-Driven Realtime Monitoring of Early Indicators for Ichthyophthirius multifiliis Infection of Rainbow Trout. Journal of Fish Diseases, 48(1), e14027. https://doi.org/10.1111/jfd.14027

Buchmann, K. (2020). Immune response to Ichthyophthirius multifiliis and role of IgT. Parasite Immunology, 42(8), e12675. https://doi.org/10.1111/pim.12675

Francis-Floyd, R., Yanong, R., & Pouder, D. (2023). Ichthyophthirius multifiliis (white spot) infections in fish. 4 p.

Guang-Ran Hu, Run-Qiu Wang, Ke Huang et al. Ecological factors associated with parasite Ichthyophthirius multifiliis gene abundance in the aquatic environment：novel clues for ecological control of Ichthyophthiriasis, 16 January 2024, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-3859658/v1]

Jørgensen, L. V. G. (2017). The fish parasite Ichthyophthirius multifiliis – Host immunology, vaccines and novel treatments. Fish & Shellfish Immunology, 67, 586-595. https://doi.org/10.1016/j.fsi.2017.06.044

Li, J., Lian, Z., Wu, Z., Zeng, L., Mu, L., Yuan, Y., Bai, H., Guo, Z., Mai, K., Tu, X., & Ye, J. (2023). Artificial intelligence–based method for the rapid detection of fish parasites (Ichthyophthirius multifiliis, Gyrodactylus kobayashii, and Argulus japonicus). Aquaculture, 563, 738790. https://doi.org/10.1016/j.aquaculture.2022.738790

Prakash, H., Sato, M., Kojima, K., Sakuma, C., Kizhakkumpat, A., Nagasawa, T., Nakao, M., & Somamoto, T. (2026). Development of a single-chain variable fragment (scFv)-conjugated affinity silk filter for the efficient removal of Ichthyophthirius multifiliis. Aquaculture, 612, 743141. https://doi.org/10.1016/j.aquaculture.2025.743141

Valladão, G. M., Gallani, S. U., Ikefuti, C. V., Levy-Pereira, N., N Rodrigues, M. V., & Pilarski, F. (2016). Essential oils to control ichthyophthiriasis in pacu, Piaractus mesopotamicus (Holmberg): Special emphasis on treatment with Melaleuca alternifolia. Journal of Fish Diseases, 39(10), 1143-1152. https://doi.org/10.1111/jfd.12447

Wang, L., Xi, B., Chen, K., Xie, J., & Pan, L. (2025). In-Situ Investigation of Copepod Predators of Ichthyophthirius multifiliis Theronts from Fish-Farming Pond. Microorganisms, 13(1), 38. https://doi.org/10.3390/microorganisms13010038

Yang, H., Tu, X., Xiao, J., Hu, J., & Gu, Z. (2023). Investigations on white spot disease reveal high genetic diversity of the fish parasite, Ichthyophthirius multifiliis (Fouquet, 1876) in China. Aquaculture, 562, 738804.

Yang, H., Shen, Y., Meng, Y., Zheng, C., & Gu, Z. (2025). Antiparasitic efficacy of procyanidins against the fish parasite, Ichthyophthirius multifiliis: In vivo and in vitro. Aquaculture, 743224. https://doi.org/10.1016/j.aquaculture.2025.743224

Milthon Lujan

Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.