Guide
Ich (ick): what it is, what it looks like, and how to treat it
One of the most common freshwater fish diseases - and one you can beat with the right approach.
- What it is: Ich (Ichthyophthirius multifiliis) is a protozoan parasite that burrows into fish skin and gills, producing the signature white spots.
- What to look for: Salt-grain-sized white spots on the body, fins, and gills; fish rubbing against decor (flashing); rapid breathing; clamped fins.
- Treat the tank, not just the fish: The parasite spends most of its life cycle off the fish - medication only kills the free-swimming stage, so treat the whole tank for the full course.
- Heat helps: Raising temperature to 28-30 °C (82-86 °F) speeds up the life cycle and shortens treatment time - but only if your fish can handle it.
- Remove carbon first: Activated carbon absorbs medication. Pull it before you dose anything.
- Quarantine prevents this: Most outbreaks trace back to a new fish added without quarantine. Four weeks is the standard minimum.
What ich actually is
Ichthyophthirius multifiliis - ich, or ick - is one of the most common diseases in freshwater fishkeeping, and one of the most mismanaged. It's a ciliated protozoan parasite, not a bacterial infection, not a fungus. That distinction matters because the wrong treatment does nothing.
The parasite has three distinct life stages, and understanding them is the key to treating effectively:
- Trophont (feeding stage): The parasite is embedded in the fish's skin or gills, feeding. This is what causes the white spots. It's also the stage least vulnerable to treatment - most medications can't penetrate the cyst surrounding it.
- Tomont (reproductive stage): The trophont drops off the fish, sinks to the substrate, and divides repeatedly inside a cyst. One tomont can produce hundreds of daughter cells.
- Theront (free-swimming stage): Newly hatched parasites swim through the water looking for a host. This is the only stage treatment reliably kills.
This is why spots seem to disappear and then come back with a vengeance when treatment stops too early. The fish looks clear while the substrate is still full of encysted tomonts, and when the next wave of theronts hatches, the fish get re-infected.
At typical tropical temperatures, free-swimming theronts die within 48-72 hours if they don't find a host - which is why treatment must maintain a therapeutic concentration throughout the entire cycle.
What ich looks like - and what it doesn't
The classic presentation is unmistakable once you've seen it: discrete white spots, each roughly 0.5-1 mm across, scattered across the body and fins. They look like someone flicked grains of salt at the fish. In early infection you might see just two or three spots. By the time a fish is heavily infected, the body can be almost completely covered.
Behavioural signs often appear before visible spots, especially when gills are involved:
- Flashing - the fish darts sideways and rubs against gravel, plants, or hardscape
- Rapid gill movement or gasping near the surface
- Clamped fins held tight against the body
- Hiding, reduced appetite, and general lethargy
Confusing ich with other conditions. Velvet (Oodinium) looks like fine gold or rust dust, not discrete white spots. Columnaris shows as white patches with ragged edges, often around the mouth. Lymphocystis produces cauliflower-like growths, not salt grains. If the spots are fuzzy rather than crisp, you may be looking at a fungal infection instead. Treating ich medication on a fungal infection wastes time and stresses your fish.
What causes an outbreak
Ich is almost always introduced - it doesn't appear spontaneously in a healthy, established tank (despite the persistent myth). The usual routes are:
- A new fish added without quarantine
- New plants or décor from an aquarium that housed fish
- Water transferred from an infected tank
Stress is the accelerant, not the cause. A fish with a compromised immune system - from temperature swings, poor water quality, overcrowding, or rough handling - will succumb faster and more severely once exposed. This is why an outbreak often follows a purchase, a tank move, or a period of water quality neglect.
Check your water parameters immediately if you spot ich. Ammonia or nitrite above zero, or a nitrate spike, tells you the fish are already fighting on two fronts. Get those right alongside treatment. See our guide to aquarium water parameters if you need a baseline to work from.
The aquarium trade sells a lot of fish in poor condition, and stores often don't quarantine stock properly. That fish you bought that "looked fine" at the shop can be carrying ich at a subclinical level - spots not yet visible - and will break out within days of the stress of bagging, transport, and re-acclimation. The four-week quarantine recommendation isn't paranoia. It's experience.
How to treat ich
Treatment works by maintaining a lethal concentration of medication in the water during the theront (free-swimming) phase, across multiple hatch cycles. There is no single-dose fix.
Step 1: Remove activated carbon
Pull any carbon from your filter before you add anything to the tank. Carbon adsorbs medication and renders your treatment ineffective. Biological media stays in - you need your cycle running. If you're unsure how your filter affects treatment, brush up on how to cycle a fish tank.
Step 2: Raise the temperature (if appropriate)
Higher temperatures speed up the parasite's life cycle, which means theronts hatch faster and the treatment is over sooner. The target range is 28-30 °C (82-86 °F). Raise temperature gradually - no more than 1-2 °C per hour - and increase aeration, because warmer water holds less dissolved oxygen.
This approach is not appropriate for coldwater species (goldfish, white cloud mountain minnows) or scaleless/heat-sensitive fish. Some discus keepers run treatment at the higher end naturally; some species of loach or pleco will struggle at 30 °C. Know your stocking list before you adjust the heater.
Step 3: Medicate
Two main chemical approaches are used in freshwater ich treatment:
- Formalin/malachite green combinations: The most effective broad-spectrum option. Products like Ich-X, Waterlife Protozin, and similar contain these. Follow the label dosing exactly. These are potent chemicals - underdosing is ineffective, overdosing is toxic.
- Copper-based treatments: Highly effective but toxic to invertebrates, scaleless fish (loaches, catfish), and many plants at therapeutic concentrations. Use only in fish-only tanks and monitor copper levels with a test kit if you go this route.
Complete the full treatment course - typically 7-10 days minimum - even after every visible spot has gone. Spots disappearing means the trophonts dropped off; it does not mean the tank is clear.
Step 4: Salt (optional, species-dependent)
Aquarium salt (sodium chloride) at 1-3 g per litre can stress theronts and support fish slime coat during treatment. It's a useful adjunct, not a standalone cure. Many fishkeepers use salt alongside medication with good results.
Do not use salt with fish that are sensitive to it: most scaleless species (corydoras, loaches, many catfish), live-bearing hybrids bred without salt tolerance, and virtually all live plants will be harmed at therapeutic concentrations. Check every species in your tank before adding salt.
Step 5: Maintain water quality throughout
Partial water changes before each medication dose remove organic waste that degrades treatment efficacy, and keep the fish in the best possible condition to fight infection. A 25-30% change before dosing is common practice. Redose after each water change according to the volume changed, not the full tank volume.
Some ich medications are hard on biological filtration. Monitor ammonia and nitrite daily during treatment - if your cycle crashes mid-treatment you have a much bigger problem. See the nitrogen cycle explained if you're not sure what to watch for.
What most ich guides miss
Gill ich is harder to spot and more dangerous
When ich infects the gills before the body, you may see a fish breathing rapidly, flashing, or hovering near the surface with no visible spots at all. It's easy to misdiagnose as a water quality issue or a bacterial infection. If your fish are showing respiratory distress and parameters are fine, treat for ich anyway - the gills are where it kills.
The tank temperature determines how long treatment takes
At 18 °C (64 °F) - the lower end for some tropical tanks - the ich life cycle can take three to four weeks to complete. At 28 °C (82 °F) it's closer to five to seven days. If you're treating a coldwater tank and stopping after a week, you may be stopping too soon. Match your treatment duration to your water temperature, not just the calendar.
Re-infection from the substrate is the most common reason treatment "fails"
Tomonts sink and adhere to substrate and décor. A gravel vacuum during water changes removes a significant number of encysted parasites before they hatch. This is an underrated step that shortens the effective treatment window.
Vacuum the substrate thoroughly before each water change during treatment. Focus on corners and under décor where tomonts accumulate. The parasites you remove mechanically are parasites that won't hatch into a fresh wave of theronts.
Not all "ich cures" are equal
Herbal or "natural" treatments - many of which are sold prominently in fish stores - have weak evidence behind them. Some may have a mild effect; none have the track record of formalin/malachite green combinations in controlled conditions. If your fish are visibly suffering, this is not the moment to experiment.
After treatment: preventing the next outbreak
Once the tank is clear, the single most effective prevention is a proper quarantine protocol for every new fish. Four weeks in a separate tank, at the same temperature you'll be treating at. Observe daily. If ich appears during quarantine, treat in the quarantine tank - your display tank stays clean.
Keep stress low. Stable temperatures, good water quality, and appropriate stocking density are what keep fish immune systems functional. If you're not sure whether your tank is appropriately stocked, the free stocking calculator is a quick check. And if you're choosing fish for the first time, look at species known to be robust - the guide to best fish for beginners covers your best options.
App-aquatic's parameter logging makes it easy to spot the conditions - temperature dips, nitrate creep - that precede outbreaks before they happen. You can log test results, set reminders, and track your full stocking list in one place.
If you had ich in your display tank and want to be certain it's clear before adding new fish, leave the tank fallow (fishless) for at least two weeks at 28 °C (82 °F). Without a host, all theronts die. It's the most reliable way to break the cycle without chemicals.
Track your parameters and get care reminders with App-aquatic.
Get the free appCan ich go away on its own without treatment?
Rarely, and not reliably. A very mild infection in a healthy, unstressed fish might resolve, but the parasite stays in the tank through its substrate stages and will re-infect when conditions allow. Treating a light infection early is far easier than managing a heavy one later. Don't wait.
My fish had ich, I treated it, and now the spots are back. What happened?
Almost certainly you stopped treatment before the full life cycle completed. The spots disappeared when trophonts dropped off the fish, but tomonts in the substrate continued to hatch. When the next generation of theronts found your fish, they re-infected. Restart treatment immediately, vacuum the substrate thoroughly, and complete the full course - at minimum seven days after the last spot disappears.
Is ich contagious to all fish in the tank?
Yes. Once ich is in the tank, every fish is at risk regardless of species. Free-swimming theronts don't discriminate. Treat the whole tank, not just the visibly infected fish.
Can I use salt to treat ich if I have corydoras or loaches?
No. Scaleless and thin-scaled fish like corydoras, loaches, and most catfish are sensitive to salt at the concentrations needed to affect ich. Use a formalin/malachite green medication at the appropriate dose for sensitive species - many product labels include a reduced dose for scaleless fish. Follow that guidance carefully.
How do I know when my tank is fully clear of ich?
Complete the full medication course as directed - typically 7-10 days - and continue daily observation for at least a week after the last visible spot. If no new spots appear and fish behaviour is normal, the tank is almost certainly clear. For absolute certainty, a two-week fallow period at elevated temperature (fishless) will eliminate any remaining parasite stages from the substrate and décor.
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