White spot disease (ich): how to recognise it and treat it
Tiny white grains on the fins and body, flashing against the gravel, then gasping — ich is the most common fish disease and one of the most curable, if you understand its life cycle.
What you are looking at
Ich — Ichthyophthirius multifiliis, white spot disease — looks like someone sprinkled the fish with salt. The spots are small, round, raised and evenly sized, roughly the size of a grain of sugar, on the fins first and then the body and gills. Before the spots are obvious the fish will usually flash: dart at the gravel or a rock and scrape itself against it, because the parasite itches. Clamped fins and quick, shallow breathing follow, and a fish whose gills are heavily infested will hang near the surface gasping.
It is a parasite, not a fungus and not a water-quality symptom, though water quality is almost always why it took hold. Ich is present at a low level in many tanks and healthy fish shrug it off. A temperature swing, a new fish added without quarantine, ammonia after a missed water change — any stress that dents the immune system — is what lets it bloom.
Two things get mistaken for it. Epistylis sits on the fish in cottony, irregular tufts rather than uniform grains, and points to dirty water rather than a parasite. Velvet is finer, gold-dusty rather than white, and is easiest to see with a torch angled across the fish. If the spots are perfectly uniform white dots, it is ich.
Why treatment fails when it fails
The spots you can see are the one stage of the parasite that no medication reaches. Each white dot is a trophont feeding under the fish’s skin, protected by it. After a few days it drops off, sinks into the substrate, and encysts; inside that cyst it divides into hundreds of free-swimming theronts, which burst out and have about two days to find a fish before they die. Only that free-swimming stage is vulnerable. Medication kills what is in the water, never what is on the fish.
This is why a treatment that “didn’t work” almost always worked exactly as designed and was stopped too early. The spots disappear when the trophonts drop off — which happens with or without medication — and the tank looks clear for two or three days. Then the cysts hatch. If the medication stopped when the spots did, the second wave meets nothing and the fish is worse than before.
The whole cycle takes roughly a week at 25 °C, longer in cooler water, shorter in warmer. Every treatment plan below is built around outlasting it.
The treatment, step by step
1. Fix the water first. Test ammonia, nitrite and nitrate today. Ich took hold because something stressed the fish, and the most common something is a reading in the red. Any ammonia or nitrite above zero means a 30–50 % water change now, and daily testing until both read zero. Nitrate above 40 ppm gets the same water change. A medicated tank with 0.5 ppm of ammonia in it is treating the symptom while feeding the cause.
2. Raise the temperature — slowly. Warmth speeds the parasite’s life cycle, which shortens the time you have to keep medication in the water and pushes the free-swimming stage out sooner. Go from your normal temperature to 28–29 °C at no more than 1 °C every twelve hours; a faster jump is its own stress. Check first that everyone in the tank tolerates it — most tropical community fish do, most cold-water fish and some catfish do not. Warm water holds less oxygen, so add an air stone or angle the filter outlet to break the surface.
3. Medicate the whole tank, for the whole cycle. Use a proper ich treatment — malachite green with formalin, or a copper-based medication in a tank with no invertebrates — dosed by the label for your actual water volume, which the app knows. Remove activated carbon from the filter, because it will pull the medication out of the water within hours. Then keep dosing on the label’s schedule for at least 10 days at 28 °C, 14 at normal temperatures, and 3 days past the last spot you see, whichever is longer.
4. Vacuum the gravel every other day. The cysts sit in the substrate. Every gravel vacuum physically removes a share of the next generation before it hatches, and doubles as the water change that keeps ammonia down. Re-dose medication for the volume you replaced.
5. Feed lightly, and watch. A fish with damaged gills is already struggling; uneaten food that rots into ammonia makes it worse. Feed once a day, what is gone in a minute. Look at the fish every morning: fewer spots, breathing slowing down, fins opening — or more spots, hiding, and gasping, which means the water, not the medication, needs attention.
In the app — run a checkup with a photo of the spots and the water readings. Finley checks the diagnosis against your actual parameters, gives the treatment as a numbered plan, and Add plan to my schedule turns it into dated tasks: the gravel vacuums, the re-dose days, the day the course ends. A follow-up check-in arrives a few days later to ask how the patient is doing, and closes the case in the journal when they pull through.
Scaleless fish, shrimp and snails
Loaches, catfish, tetras and other scaleless or small-scaled fish are sensitive to malachite green and formalin at the full label dose; the usual practice is half dose for twice the duration, and slow temperature changes matter even more for them. Copper is toxic to every shrimp and snail at any treatment dose, and it stays in the tank long after treatment. In a tank with invertebrates, treat with heat and gravel vacuuming alone, or move the fish to a hospital tank to medicate. A bare hospital tank is the best place to treat ich in any case: no substrate for cysts, easy daily water changes, and the main tank goes fish-free for two weeks, which starves every theront in it.
Keeping it from coming back
Quarantine new fish for three to four weeks before they meet the main tank. That single habit prevents most ich outbreaks, because a fish from a shop tank is a fish that recently shared water with dozens of others. Keep temperature steady — a heater that fails or a room that cools overnight is a classic trigger. And keep testing: a tank whose ammonia and nitrite stay at zero and whose nitrate stays under 20 ppm has fish with immune systems that finish ich off before you ever see a spot.
In the app — weekly water tests are on your care schedule by default, and a reading that drifts into the red gets a verdict the moment you log it, with the water change already calculated. The tank’s health score on the home screen is the early warning; ich is what happens a week after it drops.