Exactly.
From Advanced Aquarist - Marine Fish Acclimation Procedures:
Marine teleost fish (bony reef fish) readily adjust to a rapid drop in salinity without any apparent ill effects. Place fish directly into a salinity of 12-14ppt. Monitor the pH daily as this parameter has a tendency to fall in dilute saltwater.
Fish should remain in hyposaline conditions for several weeks. Marine teleost fish need more time to adjust to an increase in salinity than a decrease. Raise the salinity a few points a day until it matches your display aquarium before moving the fish to its final destination.
Contrary to the commonly held belief that a salinity lower than natural seawater is stressful to marine teleost fish (bony reef fish), reducing the gradient (difference in salinity) between the internal fluids of fish and the surrounding ambient water alleviates water and ion disturbance (Wedemeyer, 1996. Carneiro &Urbinati, 2001). Fish held in water that is close to isotonic (the salinity of the surrounding ambient water is close to matching the internal fluids of the fish) have increased stress resistance (Lim et. al, 2000). These fish also display a significantly lower mortality rate at 7 days post shipment.
I suggest placing marine teleost fish directly into a hyposaline environment during the acclimation and quarantine period (Lowry, 2004). A salinity of 12ppt (not Specific Gravity) is close to isotonic for bony reef fish. I prefer keeping the salinity at 12 to 14ppt for thirty days or more. The salinity can then be raised a few points a day until it is close to natural seawater, or matches your display aquarium.
Although studies indicate that at least some species of marine teleost fish grow faster in a salinity of 14ppt than at 35ppt (natural seawater) (Lambert, Dutil, and Munro, 1994), I do not suggest maintaining hyposaline conditions indefinitely. Do not subject marine invertebrates, sharks, rayfish, "live rock," or "live sand" to hyposaline conditions.
Hyposalinity assists marine teleost fish in recovery five ways:
Helps control external parasites
Helps fish to recover osmotic balance more quickly
Helps fish that are injured or have lost mucus protection to maintain osmoregulatory balance.
Conserves energy that can be used to recover normal homeostasis and for disease resistance
Helps fish to recover feeding behaviors more quickly
The most obvious benefit of hyposalinity therapy to marine fish, while acclimating to captivity, is that it is a proactive approach to dealing with external parasites. However, there are other significant benefits for marine teleost fish.
Osmoregulatory dysfunction is an inherent part of stress in fish. Reducing the salinity gradient between the internal fluids of the fish and the surrounding ambient water helps them to recover osmotic balance more quickly.
Injuries are a common occurrence during transport and handling. Wounds or compromises to the mucus/scale/skin barrier make osmoregulation more difficult and costly energy-wise. In marine fish, osmotic pressure can cause fluids to leak from wounds into the water. Reducing the salinity of the water decreases the osmotic pressure and loss of fluids from wounds or compromises the mucus layer.
Marine teleost fish typically consume 25 to 50% of their metabolic energy in the process of osmoregulation. Conserving metabolic energy makes more available for other processes such as regaining normal homeostasis, diseases resistance, etc.
Since osmoregulatory balance is a factor influencing feeding behaviors, it is reasonable to assume that fish that quickly recover osmotic balance will resume feeding sooner. Fish should regain osmotic balance more quickly in hyposaline conditions.
