Small white spots scattered across a fish’s body and fins, like it’s been sprinkled with salt, is the telltale sign of ich. It’s caused by a parasite (Ichthyophthirius multifiliis) that’s one of the most common diseases in both ponds and aquariums.
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What Causes It
The parasite is often already present at low levels; stress, temperature swings, or poor water quality let it multiply out of control.
Why Timing Matters
Ich has a life cycle stage that’s vulnerable to treatment and a stage that isn’t (protected on the fish itself). This is why single-dose treatments often fail and repeat dosing over 7 to 10 days is standard.
