Hyposalinity Marine Fish Treatment: Safe Salinity Reduction Protocol

Lowering specific gravity far below reef levels kills marine ich by bursting the parasite’s cells osmotically before the fish is harmed. Hyposalinity marine fish treatment holds salinity at 1.009 SG for a minimum of three weeks, a protocol popular among public aquariums because it is cheap and species-friendly for most teleosts. This guide from Gensou Aquascaping in Singapore covers how to ramp down safely, which fish should never be hypo’d, and how to return them to reef salinity without osmotic shock. Done precisely it is a strong alternative to copper; done sloppily it wastes 30 days and leaves parasites behind.

Quick Facts

• Target salinity: 1.009 SG (11-12 ppt) measured by calibrated refractometer
• Duration: minimum 21 days at target, typically 30 days to be safe
• Ramp down over 48-72 hours; raise over 7-10 days at the end
• Effective against Cryptocaryon irritans (ich) only; not velvet, flukes, or brooklynella
• Not safe for sharks, rays, most invertebrates, or live rock
• Requires hospital tank; never hypo a display with corals or inverts
• Refractometer must be calibrated with 35 ppt calibration fluid, not distilled water

Why 1.009 and Not Lower

Research on Cryptocaryon shows tomonts and theronts fail to develop below 16 ppt, but the 1.009 SG (about 12 ppt) figure provides a margin without crossing into freshwater-stress territory for most marine fish. Dropping below 1.008 increases the risk of kidney stress and secondary bacterial infection. Going above 1.012 lets a percentage of parasites survive the treatment.

Accurate measurement is non-negotiable. A swing-arm hydrometer can read 10-15% off. Use a refractometer calibrated with a 35 ppt solution from a local shop for $20-30, not DIY distilled water calibration.

The Safe Ramp Down

Start the fish in a hospital tank at normal 1.025 SG. Perform water changes with freshwater (dechlorinated, matched temperature and pH) to lower the salinity. Drop from 1.025 to 1.015 over the first 24-36 hours, then 1.015 to 1.009 over the next 24-36 hours. Faster drops stress kidneys; slower drops extend parasite survival time.

Keep pH stable with a buffer such as seachem marine buffer. At low salinity, buffering capacity drops and pH can swing dangerously.

Holding at Target

For 21-30 days, hold 1.009 SG precisely. Top off evaporation with freshwater only; top-offs with saltwater raise SG and prolong the treatment. Test salinity daily in the morning before lights come on when fish are less active.

Temperature stays at 26-27°C. Feeding continues normally; fish often eat better in hyposalinity because osmoregulatory work decreases. Nitrification bacteria tolerate the drop but work slightly slower, so monitor ammonia more frequently for the first week.

Which Fish Are Not Candidates

Elasmobranchs (sharks and rays) have different osmotic physiology and cannot be hypo’d safely. Seahorses, pipefish, and mandarins generally do poorly. Most invertebrates die below 1.020, so hyposalinity is strictly a fish-only tank treatment. For species in the unsafe list, use copper, chloroquine, or TTM instead.

Bony fish such as tangs, angels, wrasses, clownfish, damsels, and grammas handle hyposalinity well and often show faster appetite recovery than with copper.

Monitoring During Treatment

Watch for the disappearance of visible spots over the first 7-10 days. Spots fall off the fish as trophonts detach; this is expected and does not mean the cycle is complete. Continue the full duration even if the fish looks clean by day 10. Stopping early is the most common reason for treatment failure and re-infection.

If new spots appear after day 14, salinity has probably drifted upward due to inaccurate top-off. Recalibrate the refractometer and check.

Ramping Back Up

After the minimum 21-30 day hold, raise salinity slowly by swapping in saltwater at 1.025 SG during daily water changes. Aim for 0.002 SG increase per day, reaching 1.025 over 7-10 days. Rushing this stage causes osmotic shock that looks identical to disease — rapid breathing, lethargy, appetite loss.

Once at full salinity, observe for one week before moving to the display. Combine with a prazi course for flukes if the fish was wild-caught.

Why Some Aquarists Skip Hyposalinity

The protocol requires more frequent testing than copper and does not treat velvet, which kills faster than ich. Copper remains the default for fish with unknown health status because it covers both parasites. Hyposalinity is best used when a fish is copper-sensitive, or when a whole-tank ich event in a fish-only system needs treating without tearing down the rockwork.

Related Reading

Marine Ich Cryptocaryon Treatment
How to Set Up Marine Quarantine Tank
How to Quarantine Marine Fish Complete
Best Aquarium Copper Test Kit
How to Read Marine Water Test Results