Hyposalinity is a treatment regime for fish that involves a deliberate reduction in the salinity of the water ("hypo" comes from Greek term for "low" or "under"). It is most typically used as a treatment for Cryptocaryon irritans, (Marine "Ich").
Hyposalinity is not safe for use in a display aquarium containing invertebrates. If applied for treatment of diseased fish it mush be conducted in a hospital or quarantine tank, with careful control of salinity and other water parameters. A refractometer, correctly calibrated using reverse osmosis or freshwater, is required for the accurate measurement of low salinity.
For the treatment for C. irritans, the salinity has to be slowly lowered to 12 to 14 ppt (specific gravity of 1.009) over 3 days, then maintained at that level for a period of at least 4 weeks (longer than the maximum life cycle of C. irritans).