If you keep fish long enough you will deal with ich. It's not a matter of whether — it's when. The good news is it's treatable. The bad news is most people treat it wrong.
Ich (Ichthyophthirius multifiliis, or white spot disease) is a parasitic infection that looks exactly like what it sounds like: tiny white dots, roughly the size of a grain of salt, scattered across your fish's body and fins. It's the most common disease in freshwater aquariums and one of the most misunderstood.
What you're actually looking at
Those white dots aren't the parasite itself — they're cysts. The parasite has already burrowed under the fish's skin by the time you can see it. This matters for treatment because you can't kill a parasite that's inside a fish. The medication only works on the free-swimming stage of the lifecycle.
Here's what the lifecycle looks like. The cyst on the fish (the trophont) feeds and grows for several days, then drops off into the substrate. It forms a protective capsule (tomont) and divides rapidly, producing hundreds of free-swimming offspring (theronts). Those theronts have 24 to 48 hours to find a fish host before they die. That's your treatment window.
Temperature is your biggest lever
Ich reproduces faster at lower temperatures and slower at higher ones. Above 30°C (86°F) the parasite can't complete its lifecycle at all — but most fish can't handle that temperature either. The practical approach is to raise the temperature to around 28–29°C (82–84°F) for the duration of treatment. This speeds up the lifecycle, shortening how long you have to treat.
Raise the temperature slowly — no more than 1°C per hour — and make sure your tank is well aerated, since warmer water holds less oxygen.
Treatment options
Aquarium salt is the gentlest option and works reasonably well for mild infections. Use 1 tablespoon per 5 gallons, dissolved first in tank water before adding. Most freshwater fish tolerate this fine. Scaleless fish (loaches, cory catfish, certain plecos) are more sensitive — reduce the dose by half and watch carefully.
Ich-X or similar malachite green/formalin treatments are more effective for moderate to severe cases. Follow the dosing instructions exactly. These medications stain everything including silicone, filters, and your hands. Remove activated carbon from the filter before treating — it will absorb the medication.
Copper treatments are the most effective but also the most dangerous. Copper is lethal to invertebrates at any concentration and toxic to fish at concentrations only slightly above therapeutic levels. Use copper only if you know what you're doing and have a copper test kit. Never use it in a tank with shrimp, snails, or corals.
Why it probably showed up
Ich is usually introduced by new fish. The parasite can be present on a fish without visible symptoms — stress during transport brings it out. A 2–4 week quarantine tank for new arrivals is the only reliable way to prevent introduction. Most hobbyists skip this until they've lost fish to ich once. Then they never skip it again.
Stress is the other factor. Fish with a healthy immune system can suppress ich infections. Cold water, poor water quality, overcrowding, or bullying can compromise their immunity enough that a low-level infection explodes. If ich keeps coming back in your tank, the answer is usually to fix the underlying stressor rather than just treating the symptom.
Saltwater ich is different
Marine ich (Cryptocaryon irritans) looks similar but the lifecycle is longer and the treatment options are more limited. Copper is the standard treatment for reef-free tanks. In a reef tank with invertebrates you can't use copper — the main option is a hyposalinity treatment (lowering salinity to 1.009–1.010) in a separate hospital tank. This is one reason quarantine tanks are considered non-negotiable in the saltwater hobby.
During treatment
Keep up water changes — ich cysts accumulate in the substrate and vacuuming the gravel removes them before they can hatch. A 25–30% water change every other day during treatment both removes cysts and keeps water quality stable while the filter is stressed by medication.
Log your treatment in Tanqly. Track which medication you used, the dosing schedule, and when symptoms resolved. If it comes back you'll have a record of what worked and what didn't.