Disease guide
White Spot Disease (Ich): Symptoms, Treatment, and Prevention
Last updated: May 2026
- What it is: Ich (Ichthyophthirius multifiliis) is a parasitic protozoan that produces the classic white pinhead spots on fins and body. It is one of the most common diseases in freshwater aquariums and is almost always survivable if caught early.
- Treat the whole tank: The spots you see are mature parasites protected under fish skin - medication kills only the free-swimming stage. Every drop of water in the tank must be dosed, not just the sick fish.
- Temperature is your ally: Raising water temperature to 28-30 °C (82-86 °F) speeds up the parasite's life cycle, shortening the window you need to medicate. Never skip this step unless your fish can't tolerate the heat.
- Finish the full course: Stop dosing early and surviving tomonts will re-hatch and restart the cycle within days. A minimum of 10-14 days of treatment is standard; 21 days at lower temperatures.
- Epistylis is a common look-alike: Not every white spot is ich. Epistylis colonies are slightly irregular in shape and often sit on wounds. Photograph spots early so you're treating the right thing.
- Prevention is straightforward: Quarantine every new fish for four weeks before it enters your display tank. Most ich outbreaks trace back to skipping this step.
Understanding the ich life cycle - because it changes everything
Most treatments fail not because the medication is wrong, but because the keeper doesn't understand when the medication actually works. Ichthyophthirius multifiliis has three distinct life stages, and only one of them is vulnerable to treatment.
The white spots you're staring at are trophonts - feeding parasites burrowed under the fish's mucus layer. They are completely shielded from any chemical you add to the water. Once a trophont is mature, it drops off the fish and sinks to the substrate, where it becomes an encysted tomont. Inside that cyst, it divides rapidly and can produce up to 2,000 daughter cells called theronts (the free-swimming stage). Theronts must find a fish host within roughly 24-48 hours or they die. That narrow window is your treatment target.
The entire cycle runs faster at higher temperatures. At 24 °C (75 °F) it takes about 7-10 days. At 28 °C (82 °F) it compresses to 3-4 days, which is why heat is a genuine treatment tool, not a folk remedy.
A single mature ich cyst can release up to 2,000 free-swimming parasites - which is why a mild infection explodes fast if you don't catch it.
Recognising ich - and ruling out look-alikes
Classic ich presents as small, uniform, salt-grain-sized white spots - 1-2 mm across - scattered across the fins and body. Fish will often flash (rub against surfaces), breathe rapidly near the surface, or clamp their fins. In heavy infestations you'll see spots clustered on the gills before they're visible on the body, which is the most dangerous presentation because gill damage can kill fish before the disease looks severe.
Two conditions get misdiagnosed as ich regularly:
- Epistylis: Bacterial/protozoan colonies that look like slightly fluffy, irregular white tufts. They tend to sit on damaged tissue and fin edges rather than spreading uniformly across the body. Unlike ich, Epistylis won't respond to standard ich medication - it needs antibiotics.
- Lymphocystis: Viral cauliflower-like growths that are usually larger, irregular, and waxy. There's no treatment; most fish recover on their own with good water quality.
Take a clear photo of your fish against a plain background before you start any treatment. It costs you nothing and can save you from dosing the wrong medication for two weeks.
Shine a torch at an angle across your fish in low room light. Early ich spots are much easier to see when light catches them at a raking angle - you'll often spot a developing infection a day before it becomes obvious under normal lighting.
Treatment: your two main options
There is no single globally-available ich medication, because what's legal and sold varies between the US, UK, EU, and Australia. The two approaches that have solid evidence behind them are heat treatment and chemical treatment - and they are most effective combined.
Heat treatment
Raise temperature gradually - no more than 1-2 °C per hour - to 28-30 °C (82-86 °F). Hold it there for at least 10 days after the last visible spot disappears. Salt (sodium chloride, not marine salt) at 1-3 g/L can be added alongside heat to reduce osmotic stress on the parasites and support the fish's slime coat. This combination is the most fish-safe approach and works well for mild to moderate outbreaks.
Heat treatment alone is not reliable for severe infestations, scaleless fish (catfish, loaches, knifefish), or fish that can't tolerate temperatures above 26 °C - such as goldfish, white cloud mountain minnows, or hillstream loaches.
Chemical treatment
Common active ingredients include malachite green, formalin, and methylene blue. In the UK and EU, proprietary products containing malachite green (such as Waterlife Protozin or NT Labs Ich-X equivalents) are widely available. In the US, products like Ich-X (malachite green + formalin) are popular and effective. Copper-based treatments exist but are harder to dose accurately and are toxic to invertebrates and some plants.
Follow the product's dosing schedule exactly. Do not halve the dose to "be safe" - you'll kill weaker theronts and leave resistant ones. Do not overdose - malachite green is toxic to fish at high concentrations, particularly scaleless species, which typically need half the standard dose.
Activated carbon in your filter will absorb medication out of the water within hours, making your treatment completely ineffective. Remove carbon before dosing and do not replace it until treatment is finished.
The treatment timeline
Whatever you choose, structure your treatment like this:
- Day 0: Confirm diagnosis, remove carbon, begin temperature rise, start medication if using chemical treatment.
- Days 1-3: You may see more spots appearing - this is normal. Tomonts encysted before treatment started are still releasing theronts. Do not panic and re-dose.
- Days 4-7: Visible spots should begin disappearing as the treatment hits free-swimming stages and the cycle accelerates with heat.
- Days 7-14: Continue treatment even when fish look completely clear. Tomonts in substrate and decorations may still be releasing theronts.
- Day 14-21: Perform a large water change (40-50%), re-dose if your product's schedule requires it, return to normal temperature gradually.
Stopping treatment the moment spots disappear. The fish looks fine, so you stop dosing - but tomonts are still releasing theronts into the water. Within five days the infection is back, often worse than before because you've now stressed your fish through a partial treatment twice.
Water quality during treatment
Sick fish need pristine water more than healthy fish do. Ammonia and nitrite stress compromises immune function and makes it significantly harder for fish to fight off the parasites that are still attacking their gills. If your tank isn't cycled properly, sort that first - treatment in an uncycled tank is an uphill battle. Our guide on the nitrogen cycle covers what you need to know.
During treatment, do a 25-30% water change every 2-3 days. This removes free-swimming theronts mechanically, reducing the parasite load your fish are fighting, and keeps ammonia down. Gravel vacuum the substrate during every water change - tomonts settle there in numbers. Re-dose medication proportionally after each water change. Most beginners forget this and their effective drug concentration drops by a third each time they top up.
For guidance on how often to do water changes when you're not treating disease, see our water change frequency guide.
Most ich outbreaks happen in tanks with already-borderline water quality. The parasite is present in many aquariums at low levels all the time - it's stress that tips the balance toward a full outbreak. If you're getting repeat ich problems, treating the infection without fixing the underlying maintenance issue is treating symptoms, not causes.
What happens to your filter during treatment
Malachite green and formalin are both somewhat hostile to beneficial bacteria at therapeutic doses, but the effect on an established biofilter is usually manageable rather than catastrophic - your cycle is unlikely to crash. That said, monitor ammonia and nitrite daily during treatment. If levels begin to rise, increase water change frequency and back off any feeding. For more detail on keeping your cycle stable, see how to cycle a fish tank.
One thing that will kill your filter bacteria is overdosing medication. Another reason to measure your tank volume accurately before you dose - subtract the volume displaced by substrate and decor, which is typically 10-15% of the stated tank size.
Prevention: the only strategy that actually works long-term
Ich enters your tank on fish, plants, and wet equipment - almost never from dry goods. A four-week quarantine tank is the single most effective prevention tool available. It doesn't need to be elaborate: a bare-bottom 40-litre (10-gallon) tank with a sponge filter, heater, and a few pieces of PVC pipe for cover does the job. Run it at 28 °C (82 °F) to accelerate any latent ich through its cycle before the new fish ever touch your display tank.
Beyond quarantine:
- Never transfer water from a fish store bag into your tank.
- Use separate nets, buckets, and equipment for each tank - or sterilise between uses with a dilute bleach solution (rinse thoroughly).
- Keep stocking levels within sensible limits. Overcrowded tanks mean stressed fish and rapid parasite spread. Use the free stocking calculator if you're not sure where your tank stands.
- Maintain consistent temperature. Fluctuations - especially overnight drops - are a reliable trigger for ich outbreaks in tanks carrying the parasite at subclinical levels.
If you're using App-aquatic to log your water tests and tank temperatures, set a reminder to check parameters twice a week during periods of stress - like after adding new fish or moving the tank. Early detection is the difference between a minor outbreak and a tank-wide emergency.
Special cases: scaleless fish and planted tanks
Scaleless fish - including corydoras, loaches, knifefish, and most catfish - are significantly more sensitive to malachite green and formalin. Standard advice is to use half the recommended dose for these species. Heat + salt is often the safer first-line option for tanks containing them, though salt at effective concentrations (2-3 g/L) can stress soft-water species like discus and some tetras.
Planted tanks create a specific problem: malachite green can damage or kill many aquatic plants, and salt at treatment concentrations will do the same to most plants over a few weeks. If your tank is heavily planted, consider moving infected fish to a bare treatment tank rather than dosing in situ - provided you're confident no theronts have already spread to the main tank. If the main tank is already infected, you may have no choice but to treat it directly and accept some plant losses.
Track your parameters and get care reminders with App-aquatic.
Get the free appCan ich go away on its own without treatment?
Occasionally, in a very lightly infected tank with excellent water quality and low fish stress, a subclinical ich presence can remain dormant. But a visible outbreak - white spots on fish - will not resolve without intervention. The parasite reproduces exponentially; waiting it out means watching it get worse.
How long do I need to leave my tank empty to kill ich without medication?
At 24 °C (75 °F), theronts that find no host die within 24-48 hours. All stages will have cycled through in around 4-6 weeks at that temperature. At 27-28 °C (80-82 °F), that shortens to about 2-3 weeks. If you can move all fish to a separate tank, raising the empty display tank's temperature to 30 °C (86 °F) and leaving it fishless for three weeks should eliminate the parasite completely.
My fish had ich, I treated it, and now it's back. Why?
Almost certainly one of two reasons: you stopped treatment before all tomonts had completed their cycle and released their theronts, or the underlying stressor that triggered the outbreak (poor water quality, overcrowding, temperature instability) was never addressed. Treat again for the full 14-21 day course and test your water parameters carefully - start with aquarium water parameters to benchmark where you should be.
Is ich contagious to humans or other pets?
Ichthyophthirius multifiliis is a fish-specific parasite. It cannot infect humans, mammals, reptiles, or birds. You can safely work in an infected tank - just wash your hands after, as you should after handling any aquarium water.
Do I need to treat all tanks in my fish room if one has ich?
Only if there has been shared water, shared equipment, or fish transferred between the infected tank and other tanks since the outbreak began. Ich does not travel through the air. If your tanks are genuinely isolated - separate equipment, no transfers - there is no need to medicate healthy tanks prophylactically.