Hyposalinity or Osmotic Shock Therapy for Marine Ich: A Careful, Copper-Free Option

In this guide

Hyposalinity means deliberately lowering the salinity of a treatment tank to a level that marine parasites like Cryptocaryon irritans (marine ich) cannot survive, while the fish itself, being far more tolerant of salinity change, rides it out. It is one of the few effective ich treatments that does not involve copper, which makes it valuable for fish species that react badly to copper medication, such as certain wrasses, sharks and rays.

The science behind it

Cryptocaryon is an obligate marine parasite. It cannot regulate its internal salt balance the way a fish can, so when the surrounding water is diluted well below normal seawater levels, the parasite's cells take on water and rupture. Marine fish, by contrast, actively pump salt to maintain their internal balance and can tolerate a specific gravity as low as 1.009 for weeks without harm, provided the change is made slowly.

How to do it safely

• Move the fish to a bare-bottom quarantine tank, never the display tank, since invertebrates and coral cannot survive low salinity
• Lower specific gravity gradually from the normal range of 1.020 to 1.025 down to 1.009, over 48 hours, never faster
• Maintain 1.009 for a minimum of 3 weeks to break the parasite's full life cycle
• Monitor pH and alkalinity closely, since the diluting water is often low in buffering capacity and can drag pH down along with salinity
• Use a refractometer, not a swing-arm hydrometer, since accuracy matters a great deal at these low readings
• Raise salinity back up just as slowly once treatment is complete

What can go wrong

The most common mistake is dropping salinity too fast, which stresses the fish more than the disease does. The second is using plain tap water without adding back carbonate hardness and trace minerals, which leaves the water soft and unstable. Sharks, rays and some sensitive invertebrate-adjacent fish should not undergo hyposalinity at all without veterinary guidance, since their osmoregulation works differently.

A practical note for Indian tanks

Refractometers are now easily available through Indian aquarium importers and online marine supply stores, and this is one of the few pieces of equipment worth buying rather than borrowing, since getting the specific gravity wrong in either direction defeats the entire treatment.

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