Ichthyophthirius Multifiliis in Fish

Ich, or White Spot, Will Eventually Kill Fish

Ichthyophthirius multifiliis white spot treatments for fish

The Spruce / Kaley McKean

Ich is caused by an external parasite that causes multiple white spots on your freshwater fishes' skin and gills. This is a common parasitic infection of freshwater fish and is one of the few fish parasites that can be seen with the naked eye. However, there are other non-parasitic causes of white spots on fish that need to be ruled out before treatment is initiated. Understanding the parasite's life cycle is critical to successful treatment.

What is Ich?

"Ich" or "white spot disease" is caused by the protozoan parasite, Ichthyophthirius multifiliis, which means in Latin "Fish Louse with many children." The saltwater form of ich or white spot disease is caused by Cryptocaryon irritans. Both parasites have a complex life cycle that makes them difficult to treat. The large feeding (trophont) stage is visible to the naked eye as the white spots on the fish’s body, but highly resistant to treatment. The free-swimming theront stage is not visible, but is the only stage in the parasite’s lifecycle susceptible to treatment. It only takes one trophont to reproduce after feeding on a fish (as a tomont), releasing 1,000 new infective organisms (the theront) into a system, so infestations can occur very rapidly. The life cycle is temperature dependent, with fewer days between stages in warmer water, and severe delay in colder water.

Signs of Ich in Freshwater Fish

  • Small white spots on the fish's body
  • Fish flashing, or using objects in their environment to scratch themselves
  • Bruising or scale loss secondary to flashing
  • Lethargic and increased respiratory effort
  • Sudden death (can be multiple fish in one system)

White spot disease can be mistaken for other non-serious issues. Fin ray fractures, or fractures to the cartilage of a fin, can look very similar, but are not life threatening. Breeding tubercles on male goldfish, producing multiple white bumps on the operculum and pectoral fin, also look identical to white spot disease, but are normal anatomical variations. Lymphocystis, a viral disease in fish, can produce similar white bumps but can be differentiated by your veterinarian.

Causes of Ich

The most common cause of Ich is failure to quarantine a new fish addition to the aquarium. Since it only takes one infectious Ich parasite to reproduce and then spread through an entire tank or pond, most fish will "look okay" and not act sick at all until a few life cycles of the parasite are complete, which can take a few days to a few weeks, depending on your water temperature. Safely quarantining all new fish will prevent the spread of Ich to your main aquarium.

Additional causes may include:

  • Using infected equipment between tanks without proper sanitation
  • Transferring infected filter media or décor between tanks
  • Moving infected water between systems


For treatment to be successful, you may have a veterinarian to examine your sick fish to make a correct diagnosis. Remember, there are other things on fish that can look remarkably similar to white spot disease that will require different treatment. Once a diagnosis is confirmed, there are treatment options available for prescription through your aquatic veterinarian and fish store. Monitoring and maintaining your water temperature is critical to ensure the parasite is completely eliminated from your system. Many over-the-counter "treatments" do not take this into account and if not properly used can severely hurt your fish in addition to not completely treating the Ich.

Many online forums will recommend manipulating your tank temperature to speed up or immobilize Ich. Unfortunately, in doing so, you will stress out your fish and make them more susceptible to death from secondary causes.

How to Prevent Ich

  • Quarantine new fish for four to six weeks (temperature dependent)
  • Quarantine new invertebrates previously kept with fish for two to four weeks
  • Quarantine all new plants before adding to the tank (two weeks with no fish)

To prevent Ich or many other parasites and diseases from entering your aquarium, all new additions, including fish, invertebrates and plants, should be quarantined in a separate tank using separate equipment for four to six weeks. Quarantine will be slightly shorter at higher temperatures. Do not manipulate your fish's ideal temperature range in order to shorten your quarantine period. This can stress out your fish and make them susceptible to many diseases and parasites.

All new plants previously kept with fish should be quarantined. By keeping plants isolated from all fish and inverts for at least two weeks, the parasite life cycle will break and the parasite will die off. Ich requires a fish host to complete its life cycle. Use these two weeks to beef up your plants with some extra fertilizer since transport and handling can easily damage aquatic plants.

In order to improve your fish's overall health and wellbeing, be sure to maintain good water quality at all times and feed an appropriate diet. Keep up with a regular maintenance schedule. Check in on all your fish on a regular basis and understand their normal appetites and behaviors so you can quickly judge when something is wrong. If you suspect something is wrong with your fish, contact your aquatic veterinarian as soon as possible.

Article Sources
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  1. Francis-Floyd, Ruth. Ichthyophthirius Multifiliis (White Spot) Infections In Fish1University Of Florida IFAS Extension, 2020