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White spot is the most common disease in aquarium fish, and almost every fishkeeper meets it sooner or later. The good news is that it is very treatable when caught early. The key is understanding the parasite's life cycle, because treatments only work on one stage of it.
This guide covers freshwater white spot (Ichthyophthirius multifiliis, usually called ich) and its marine equivalent (Cryptocaryon irritans, often called marine ich). They look similar but are different parasites and are treated differently.
How to recognise white spot
- Small white dots, about the size of a grain of salt, on the body, fins and gills.
- Fish flicking or rubbing against rocks, decor or the substrate.
- Clamped fins, faster breathing and fish hanging near the surface or filter outlet.
- Loss of appetite and lethargy in heavier infections.
Don't confuse white spot with the larger, irregular white tubercles that male goldfish develop in breeding season, or with fungus, which looks fluffy rather than like distinct grains. Marine velvet (Amyloodinium) looks like a fine gold or grey dust and progresses much faster than marine white spot, so it needs urgent treatment.
Why white spot appears
The parasite is very often introduced with new fish, plants or water from another tank. It can also be present at low levels and flare up when fish are stressed. Common triggers include:
- Adding new fish without quarantine.
- Sudden temperature drops, for example a failed heater or a large cold water change.
- Poor water quality, especially ammonia or nitrite.
- Bullying, overcrowding or transport stress.
Understanding the life cycle
This is why white spot sometimes seems to vanish and then return worse. The visible spots are the parasite feeding under the fish's skin, where treatments cannot reach it.
- Feeding stage (trophont): the white spot you can see, protected by the fish's skin.
- Cyst stage (tomont): the mature parasite drops off, settles on the substrate or decor and divides into hundreds of new parasites.
- Free-swimming stage (theront): the new parasites swim off to find a host. This is the only stage that medication kills.
The whole cycle speeds up in warm water and slows right down in cold water. At 25–26°C a freshwater cycle can take only a few days; in a cold pond it can take weeks. Marine white spot has a longer cycle, and the cysts can take up to two to three weeks to hatch.
How to treat freshwater white spot
- Test your water first. Check ammonia, nitrite and nitrate with a kit such as the NT Labs Aquarium Lab Freshwater Multi Test Kit. Medication works poorly in polluted water, so do a water change first if levels are high.
- Remove chemical filtration. Take out activated carbon, Purigen and similar media, as they will absorb the medication. Keep biological media running.
- Treat the whole tank. Every fish has been exposed, so dose the display, not just the fish with visible spots. A dedicated white spot treatment such as eSha Exit or API White Spot Cure is the first choice. If you also see fin damage or secondary infection, eSha 2000 is a broad-spectrum option. Always follow the dose on the label and check it is safe for any shrimp, snails or sensitive species.
- Raise the temperature slightly for tropical fish. Raising the temperature gradually to around 27–28°C (only if all your species tolerate it) speeds up the life cycle so the parasites reach the vulnerable free-swimming stage sooner. Add extra aeration, because warm water holds less oxygen. A reliable adjustable heater such as the Eheim ThermoControl makes this easier to control.
- Keep treating after the spots go. Continue the full course and for several days after the last spot disappears. Stopping early is the most common reason white spot comes back.
- Siphon the substrate. Gravel cleaning during treatment removes many of the cysts sitting on the bottom.
How to treat marine white spot
Marine white spot is harder to clear because most effective treatments, such as copper, are toxic to corals and invertebrates. This means treatment usually happens in a separate tank.
- Move fish to a hospital tank. Treat the fish in a bare quarantine tank with a heater, air and a mature sponge filter. Our guide to setting up a quarantine tank explains how.
- Use a reef-safe treatment only where appropriate. Products such as MinnFinn Marine & Reef are designed as a short bath for fish. Follow the instructions carefully and read the label for compatibility.
- If using copper, test it. Copper must be held within a narrow range to work without harming fish. Use a dedicated test such as the Seachem MultiTest Copper Test Kit or Hanna High Range Copper Checker. Never add copper to a display with corals or inverts.
- Leave the display fishless. With no fish to host them, the parasites in the main tank die off. Most aquarists leave the display without fish for around 72 to 76 days to be confident the cycle is broken.
For suspected marine velvet, act immediately; eSha Oodinex is formulated for velvet and other marine protozoan parasites.
White spot in ponds
Pond fish get white spot too, usually in spring when the water is warming and the fish's immune systems are still weak. Use a pond treatment such as Tetra Pond MediFin, dosed for the full pond volume, and remember the life cycle is slow in cool water so treatment takes longer. See all fish disease treatments.
Supporting recovery
- Keep water quality excellent with small, regular water changes, re-dosing medication for the volume replaced if the label says so.
- Feed lightly with high-quality food so you don't add to the waste load.
- A UV steriliser such as the Fluval UVC In-Line Clarifier can reduce free-swimming parasites passing through it. It helps to prevent outbreaks but isn't a cure on its own. Browse UV sterilisers.
How to prevent white spot
- Quarantine all new fish for two to four weeks before adding them to your display.
- Keep temperature stable. Check heaters regularly and match the temperature of new water.
- Avoid overcrowding and incompatible tankmates.
- Keep up regular maintenance so water quality never drifts.
For other conditions, from fin rot to fungus, read our complete guide to common aquarium fish diseases, or browse all treatments and dips. If you're unsure what you're seeing, send our team a photo and we'll help you identify it.
