Koi Ich (White Spot): Treatment Guide

By KoiQuanta Editorial Team|Updated Mar 2026

Ich (Ichthyophthirius multifiliis) is visible, recognizable, and treatable - but only if you hit it at the right lifecycle stage. That's the catch. The white spots you see on a fish are the trophont stage, embedded in the skin and completely immune to chemical treatment. You can dose the tank and kill nothing because you're treating at the wrong time.

Understanding the lifecycle is what makes treatment work.

TL;DR

Duration: 5-7 days at 68°F, longer at lower temperatures.
Duration: 2-4 days at 68°F (up to weeks at lower temperatures).
Lifespan without finding a host: 24-48 hours.
Raise salt to 0.5% (4.2 lbs per 100 gallons, added gradually over 48 hours) 2.
Raise temperature to 78-80°F (26-27°C) over 48-72 hours 3.
Standard dose: 0.1 ppm in ponds, 0.15 ppm in quarantine tanks.
Repeat every 48 hours for 3-4 treatments.

The Ich Lifecycle - Why Timing Matters

Ich has three stages:

Trophont (on the fish): The mature parasite embedded under the skin epithelium. The white spots you see. NOT susceptible to chemical treatment. Duration: 5-7 days at 68°F, longer at lower temperatures.

Tomont (encysted, off the fish): The trophont drops off the fish, attaches to substrate or tank walls, and divides internally into hundreds of infectious free-swimming cells. NOT susceptible to most chemical treatments. Duration: 2-4 days at 68°F (up to weeks at lower temperatures).

Theront (free-swimming): The infectious stage, actively seeking a host. SUSCEPTIBLE to chemical treatment. Lifespan without finding a host: 24-48 hours. This is your treatment window.

The practical implication: you need treatments active in the water during the theront release window, and you need to maintain treatment through multiple reproductive cycles to catch all emerging theronts.

Recognizing Ich

Small white spots, resembling salt crystals or grains of white sugar
Evenly distributed across fins, body, and sometimes gills
Fish showing flashing, scratching, and irritation behaviors
Elevated gilling (gill involvement indicates heavy infection)
Lethargy and reduced feeding in advanced cases

Don't confuse ich with velvet (smaller, gold dust appearance, more densely distributed), anchor worm eggs, or excess mucus. Get a good look under bright light. White, discrete spots that are clearly below the surface layer of the skin = ich.

Treatment Options

Salt Plus Heat

The most accessible treatment without specialty medications.

Protocol:

Raise salt to 0.5% (4.2 lbs per 100 gallons, added gradually over 48 hours)
Raise temperature to 78-80°F (26-27°C) over 48-72 hours
Maintain for minimum 7-10 days at target temperature and salt

Why it works: Elevated temperature shortens the free-swimming theront window (they die faster without finding a host), and the entire lifecycle completes faster - which means your treatment needs to be active over a shorter total period. Salt reduces osmotic stress on the fish while parasites are present.

Caution:

This treatment conflicts with KHV observation temperature requirements (68°F). If you're simultaneously watching for KHV, you have a conflict. Prioritize based on risk - for domestic fish with low KHV risk, heat treatment is reasonable.
Monitor dissolved oxygen closely. Higher temperatures hold less dissolved oxygen, and stressed fish need more. Max aeration.
Don't heat above 82°F - koi begin experiencing heat stress at this level.

Malachite Green and Formalin

These are the most effective chemical treatments for ich and the ones professional operations have relied on for decades.

Malachite green: Effective against theronts (free-swimming stage). Standard dose: 0.1 ppm in ponds, 0.15 ppm in quarantine tanks. Repeat every 48 hours for 3-4 treatments.

Note: Malachite green is carcinogenic and handling requires gloves, eye protection, and care. It's also not FDA-approved for fish intended for human consumption (not relevant for koi, but worth noting). The crystal form and the solution form require different dose calculations.

Formalin: Effective against all external parasites including ich theronts. Standard prolonged bath: 25 mg/L (1 mL of 37% formalin per 40 liters). 4-6 hour exposure, then partial water change.

Important: Formalin consumes oxygen. Run maximum aeration during and after treatment. Have a backup aeration source ready. Watch fish closely for the first hour - if they're struggling to breathe, do a water change immediately.

Commercial Products

Several commercial products combine malachite green and formalin (or malachite green alone) in formulations designed for pond or tank use. API's Pond Salt, Jungle Pond Salt, and various "ich treatments" fall in this category. Follow label dosing exactly and note that most require repeat treatment every 48-72 hours.

Treatment Schedule

A complete ich treatment protocol:

Day 1: Identify ich. Begin temperature increase (target 78°F). Begin salt increase (target 0.5%).

Days 2-4: Maintain elevated temperature and salt. Watch fish closely. The white spots are still present and may even look more prominent as the trophonts mature and prepare to drop.

Day 5-6: Spots begin dropping off fish (trophonts leaving). Theront release begins. This is when chemical treatment is most effective if using malachite green or formalin - dose at this point.

Days 6-14: Continue treatment. Repeat chemical doses every 48-72 hours. Maintain elevated temperature. Perform water change before each chemical redose.

Day 14+: If no spots visible for 7 days and fish are behaving normally, begin gradual temperature reduction and salt reduction. Don't stop treatment abruptly.

Common Treatment Mistakes

Stopping treatment when spots disappear. The spots disappearing means trophonts dropped off - not that the parasite is gone. Theronts are now emerging from the encysted stage. This is exactly when you need treatment active.

Treating at the wrong temperature. Ich treatment is significantly less effective at low temperatures because:

The parasite lifecycle slows, extending the period you need treatment active
Fish immune response is suppressed in cold water
Some chemical treatments are less active at lower temperatures

Inadequate aeration during formalin treatment. Formalin depletes oxygen rapidly. Fish die from asphyxiation, not disease. This is a manageable risk with adequate preparation, but it requires attention.

Treating only the fish. The parasite completes most of its lifecycle off the fish. Treating only the fish (dips rather than tank treatment) doesn't address the reservoir of encysted parasites on the tank walls and floor.

FAQ

How long does ich treatment take for koi?

A complete ich treatment cycle takes 14-21 days at elevated temperature. At lower temperatures (below 65°F), the lifecycle slows and the total treatment window extends to 3-4 weeks or more. The treatment isn't "done" when the spots disappear from the fish - it's done when 7 days have passed without new spots after the treatment window has covered multiple lifecycle cycles.

Can I treat ich in a pond with other fish?

Yes, but it's harder than treating in a quarantine tank. You need accurate volume calculations for dosing, you can't easily control temperature in a large outdoor pond, and you may need to remove plants before using chemical treatments. If you catch ich in a display pond, do water changes to reduce parasite load, maximize aeration, and use salt plus heat if you have a heating system. Consider whether isolating affected fish is practical for a display pond treatment.

Does salt cure ich in koi?

Salt alone at 0.3% does not effectively treat ich. At 0.5% combined with heat (78-80°F), salt dose calculator can be effective by shortening theront lifespan and accelerating lifecycle completion through the treatment period. But salt at standard quarantine concentrations (0.3%) is not a reliable ich cure - it provides osmotic support but not adequate antiparasitic action against ich specifically.