Hyposalinity exploits osmoregulation differences between fish and parasites. Protocol: 1) Move fish to bare-bottom QT with sponge filter. 2) Drop salinity from 35 ppt to 16 ppt over 24 hours. 3) Continue dropping to 12 ppt over the next 24-48h (1.009 SG). 4) Hold at 12 ppt for 14-21 days. 5) Slowly raise back to 35 ppt over 2-3 weeks before transferring to display. Why it works: Cryptocaryon irritans (marine ich) cannot survive below 16 ppt - osmotic pressure ruptures cells. Most marine teleost fish tolerate 12 ppt because they actively regulate internal salt balance. What it kills: ich, velvet (some strains), brooklynella. What it does NOT kill: some bacterial infections, flukes (still need prazi), certain parasites. Risks: stresses fish, can damage scaleless species (eels, sharks), kills any inverts present, kills coral. Use ONLY in dedicated QT - never in display. Alternatives: copper at 2.0 ppm Cu (effective + safer for fish but kills inverts), tank transfer method (TTM, no medication), formalin baths.
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