Ich (White Spot Disease) in Fish: Symptoms, Life Cycle, Treatment

Ich, also called white spot disease, is the most common illness in freshwater aquariums. Almost every fishkeeper runs into it sooner or later, especially after adding new fish or after a sudden temperature drop. The good news is that when it is caught early and treated properly, it can be cured in the vast majority of cases. In this guide we explain what it is, how its life cycle works (the key to treating it), what the symptoms are and how to treat it step by step.

What ich is

Ich is caused by a parasitic protozoan, Ichthyophthirius multifiliis, which lodges under the skin and in the gills of fish. The white dots you see, about the size of a grain of salt or sugar, are parasites encapsulated in the fish's tissue. It nearly always appears when two things happen together: the parasite enters the tank (with fish, plants or water from another tank) and the fish are stressed or run down from shipping, temperature swings or poor water quality. That is why the best prevention is proper acclimation with quarantine.

The life cycle: why treatment takes time

Understanding the cycle is what separates a treatment that works from one that fails. The parasite goes through three stages:

1. Trophont (on the fish): it lives under the skin and feeds. This is the visible white spot. At this stage medications cannot reach it, because it is shielded by the fish's tissue.
2. Tomont (on the bottom): once mature, it drops off the fish, settles on the substrate or plants and forms a cyst in which it divides into hundreds or thousands of new parasites.
3. Theronts (free swimmers): they burst out of the cyst and swim in search of a host. If they do not find one within a day or two, they die. This is the only stage vulnerable to treatment.

How long the cycle takes depends heavily on temperature:

Water temperature	Approximate cycle length
59°F (15°C)	2-3 weeks or more
70°F (21°C)	About 1-2 weeks
75-79°F (24-26°C)	About 4-7 days
82-86°F (28-30°C)	About 3-4 days

That is why treatment has to last several days: you need to wait until every parasite on the fish has dropped off and reached the free-swimming stage. And that is why you treat the whole tank, not just the spotted fish: cysts sit on the bottom and theronts are throughout the water, even if other fish show no symptoms yet.

Symptoms of ich

• Small, round white spots on the fins, body and gills, as if the fish had been sprinkled with salt.
• Fish flashing or rubbing against rocks, driftwood or the substrate.
• Clamped fins, lethargy and loss of appetite.
• Rapid breathing or gasping at the surface when the gills are affected.
• In advanced cases, cloudy-looking skin or excess slime.

Do not confuse it with breeding tubercles on some fish (such as male goldfish in spawning season) or with velvet (oodinium), which looks like a finer, gold-colored dust. If you are unsure, check our guide to fish diseases.

Step-by-step treatment

1. Remove activated carbon (and other chemical media) from the filter, because it pulls medication out of the water. Keep the filter running with its biological media.
2. Do a 25-30% water change and vacuum the substrate to remove some of the cysts.
3. Raise the temperature gradually, if your fish can handle it: about 2°F (1°C) every 12-24 hours up to 82-86°F (28-30°C). This speeds up the parasite's cycle and shortens treatment. Use a reliable aquarium thermometer to keep track.
4. Increase aeration: warm water holds less oxygen, and fish with damaged gills struggle to breathe. An airstone on one of the air pumps we recommend helps a lot.
5. Dose an ich-specific medication (API, Seachem, Hikari, Tetra and others make them, based on malachite green, formalin, copper or other active ingredients). Follow the label's dose and schedule exactly.
6. Keep treating for several days after the last spot is gone, as the manufacturer directs (often 3 days or more), because cysts may still be hatching on the bottom.
7. When you finish, do a large water change, put fresh carbon back for a few days if you want to remove residue, and bring the temperature down gradually to its normal level.

When not to raise the temperature

Going up to 82-86°F (28-30°C) is not right for every tank. Coldwater fish (goldfish, White Cloud minnows), panda corys and other species that prefer cooler water, or tanks with shrimp may not tolerate it well. In those cases keep the temperature within the species' range and rely on the medication, even if treatment takes longer. Review the ranges in our guide to aquarium temperature.

Precautions: scaleless fish, invertebrates, plants and salt

Not every tank inhabitant tolerates treatment equally:

• Scaleless or fine-scaled fish (loaches, corydoras, plecos, knifefish, small tetras): they tend to be more sensitive to medications. Many labels call for a reduced dose with them. The clown loach is notorious for catching ich easily and for being delicate with treatments.
• Shrimp and snails: copper and many other active ingredients are toxic to invertebrates. If you keep cherry shrimp or other inverts, make sure the product is invertebrate-safe or move them to another tank during treatment.
• Salt: non-iodized aquarium salt is sometimes suggested as support, but it has drawbacks. Many plants do not tolerate it, and loaches, corydoras and other scaleless fish are sensitive to it. If you use it, do so only with species that tolerate it, at low doses, and never without researching first.
• Plants: some medications harm delicate plants. Read the label.
• Filter bacteria: do not clean the filter during treatment.

Measure	Benefit	Caution
Raising temperature	Shortens the parasite's cycle	Only if your species tolerate it; add aeration
Ich medication	Kills theronts	Follow the label; care with inverts and scaleless fish
Salt	Support in some cases	Bad for many plants and scaleless fish
Water changes and vacuuming	Removes cysts and improves water	Match temperature and use conditioner

Prevention: quarantine and a stable tank

The most effective way to avoid ich is not to bring it in and not to give the parasite stressed fish:

• Quarantine all new fish for 2-4 weeks in a separate tank.
• Never pour store water into your tank when acclimating.
• Stable temperature: a properly sized heater (roughly 4 watts per gallon, or 1 W per liter) prevents sudden drops that weaken fish.
• Good water quality: ammonia and nitrite at 0 and weekly 20-30% water changes.
• Avoid stress: sensible stocking, compatible species and hiding places. More ideas in why are my fish dying.

Common mistakes when treating ich

• Stopping treatment once the spots disappear: cysts on the bottom reinfect the fish days later.
• Treating only the affected fish in a separate container and leaving the tank untreated.
• Leaving activated carbon in the filter.
• Raising the temperature all at once, or doing it with species that cannot handle it.
• Using full doses with scaleless fish or invertebrates without reading the label.
• Mixing several medications at the same time.
• Forgetting extra aeration in warm water.

With patience, a full course of treatment and some prevention, ich stops being a serious threat to your aquarium.

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