Treating Ich: Heat, Salt or Medication, and the Temperature Ceiling That Decides

Ich is the disease every freshwater fish health guide describes, and most of them give the same first instruction: raise the temperature. Two readers caught the problem with that. The bristlenose pleco guide said to raise the temperature and never said to what, against a setup guide that tops out at 80°F, and the zebra danio guide said the same for a fish whose own setup guide runs without a heater. The number that stops ich is above both ceilings. This page is where the contradiction gets settled: what the parasite does, why the timing matters more than the medicine, which fish can take the heat and which cannot, and what the rest get instead.

What Ich Does, and Why Only One Stage Can Be Killed

Ichthyophthirius multifiliis has three stages, and the University of Florida's fact sheet by Francis-Floyd, Yanong and Pouder lays them out. The trophont is the white spot: it invades and encysts between the outer layers of the skin and gills and feeds on those tissues. When it matures it leaves the fish as a tomont, drops to the bottom and quickly secretes a gelatinous-walled cyst, inside which it divides up to 10 times into as many as 1,024 daughter parasites. Those burst out as theronts, the free-swimming stage that must find a fish within a day or so, and the cycle starts again.

The line that decides every treatment is this one: the free-swimming phase is unprotected and therefore highly susceptible to chemicals, and treatment protocols should be designed to target that theront stage. The spot on the fish is armored by the fish's own skin. The cyst on the gravel is armored by its wall. Nothing you add to the water touches either. You are treating the water to catch the parasite in the hours it spends swimming between the two.

Temperature sets how often that window comes round. At 75 to 79°F the whole cycle completes in about 3 to 6 days. At 60°F it stretches out until treatments are spaced 3 to 5 days apart and the course runs for weeks. That is the real reason to warm a tank during treatment, whatever the method: a faster cycle means more theronts exposed sooner and a shorter course.

Heat: The Method With a Ceiling

VCA's guidance is the one the species guides were paraphrasing: most strains of ich cannot survive in water above 86°F, so slowly raising the tank to that temperature is enough to get rid of the parasite, and adding oxygen to the water is essential for the method, because warm water holds less. VCA also says what the species guides left out: not all fish tolerate that heat.

That is the whole question for a keeper. Heat alone is a treatment only for a fish whose tolerated range includes 86°F. For everything else, heat is a way to shorten the cycle, taken only as far as the species ceiling allows, and something else does the killing.

Raise it slowly, a couple of degrees a day, and add an air stone the day you start. The zebra danio health guide has the species version of the rule: a tank run without a heater gets one for the length of the treatment, set to the ceiling, not to 86. For a species that lives at 86 already, the discus tank setup guide covers the two-heater setup that keeps it there.

Salt: For Scaled Fish Only

VCA's salt dose is about a tablespoon of aquarium salt per 5 gallons, added with time for the fish to acclimate, and it comes with a hard limit: the method is only suitable for scaled fish. The Florida fact sheet gives the same treatment in the units a lab uses, 4 to 5 grams per liter as a prolonged bath for 7 to 10 days, and notes it works at the warmer temperatures. Aquarium Co-Op adds the hobby's version of the caution: catfish and loach species can be more sensitive to salinity changes.

So salt is the route for a goldfish, a koi in a treatment tub, a livebearer, a betta or a tetra tank. It is not the route for corydoras, plecos or loaches, and it is not for a tank with shrimp in it. The quarantine and treatment guide covers dissolving it first and dosing gradually.

Medication: Malachite Green, Formalin and What to Skip

For the fish that can take neither heat nor salt, and for any outbreak that is already heavy, the medicine is the answer. Aquarium Co-Op's protocol uses a malachite green product at 5 ml per 10 gallons, dosed every 24 hours until no signs of ich remain, then once more, with a one-third water change before each dose and the same concentration added back. Its two cautions are worth keeping: do not halve the dose for sensitive fish, since half-strength is not strong enough to kill ich, and if there is no improvement after 5 days the disease was most likely misdiagnosed.

Formalin is the other standard. The Florida sheet gives 25 mg/L, which is 1 ml per 10 gallons, with a half dose of 12.5 mg/L for sensitive species, and the quarantine guide carries the Merck warning that vigorous aeration is essential while it is in the water. Copper sulfate works but its dose is calculated from the tank's total alkalinity, which most home keepers never test, and copper is lethal to every aquarium shrimp. The cherry shrimp health guide treats a copper-based fish medication as the most common way a shrimp colony dies. Move shrimp and snails out before any medication goes in.

During Treatment

• Vacuum the gravel with each water change. The cysts sit on the bottom. A gravel vacuum removes some before they hatch, and the daily or alternate-day change the medication needs is the moment to do it. The water changes guide covers matching the temperature of the new water, which matters more than usual in a tank being held warm.
• Pull the carbon. Activated carbon in the filter removes the medication as fast as you add it.
• Aerate. Warm water and formalin both cut dissolved oxygen. An air stone runs for the whole course.
• Count the fish every day. Ich kills through the gills, and a fish breathing fast at the surface with no spots visible is the gill infection. The sick fish guide covers reading that.

After: The 30 Days

VCA's line on the aftermath is the one that stops the second outbreak: ich is very contagious, so quarantine infected fish for at least 30 days before introducing any new fish to their tank. That is also the quarantine every new fish gets before it ever meets the display, which is where nearly every ich outbreak starts. A tank that has been through a full course with no fish in it for a week is clean, since the theronts die without a host, and a tank that skipped the course and moved the spotted fish out is not.

The Takeaway

Find your species on the table. If its ceiling includes 86°F, warm the tank there slowly with an air stone and hold it for two weeks. If it does not, warm only to the ceiling, then salt a scaled fish at a tablespoon per 5 gallons, or medicate a catfish, pleco, loach or heavy outbreak with malachite green at full strength. Vacuum the gravel with every change, keep going until the deaths stop, and quarantine everything new for 30 days after.

Printable Guide

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Sources & Further Reading