While many saltwater fishes can survive a variety of salinity levels in the wild, the home aquarist should be careful to maintain consistent salinity in a saltwater aquarium for optimal animal health. However, many marine fishes can tolerate low salinity (hyposalinity) without adverse effects, and there are a few situations in which a hyposaline environment is actually beneficial in an aquarium setting.
What is Salinity and How Do You Measure It?
Salinity is the measure of the amount of dissolved salts or "saltiness" in water. Salt water is denser than freshwater, and the measurement unit most commonly used by aquarists in North America is called specific gravity. Tropical marine aquarists generally maintain a specific gravity of between 1.021 and 1.025 in a home aquarium, meaning the water in their aquaria is 1.021 to 1.025 times denser than freshwater. Specific gravity is most commonly measured by aquarists with either a refractometer or a hydrometer, although some situations call for a more precise measurement tool.
Effects of Low Salinity on Fishes
Fish must maintain a balance between salt and water within their body, which they do through a process called osmoregulation. As such, most fishes can adapt to different salinities. In general, moving a fish from a normal salinity environment to a hyposaline environment (say 1.010) is easier on the animal because the fish does not have to work as hard to actively expel salt from his body. Going the other direction is far more stressful on the fish, and can result in injury or death if not done gradually or the difference in salinities is too much.
Therapeutic Effects of Low Salinity on Marine Fishes
Because a marine fish needs to work less to maintain a balance between salt and water when in a less saline environment, hyposalinity can be used to reduce stress on aquarium fishes. Placing a stressed, sick or injured fish in a hospital tank with a specific gravity as low as 1.008, for example, can help the fish recover faster. Another therapeutic use of hyposalinity is to kill external parasites such as Cryptocaryon irritans ("marine ich").
Treating a fish with hyposalinity is commonly referred to as osmotic shock therapy. While a fish may be moved from regular salinity to hyposalinity without acclimatization, the converse is not true. A good rule of thumb for appropriate acclimatization is replacing a quarter of the hospital tank's hyposaline water with salt water matching the destination aquarium's salinity daily until the two tanks have the same salinity.
Maintaining a Consistent Salinity in a Saltwater Aquarium
Overall, consistency is the most important factor in terms of fish health in a saltwater aquarium. Keeping a consistent salinity within the acceptable range is far more important than hitting a specific number. Having said that, hyposaline conditions (as low as 1.008) in a hospital tank can be very useful for treating sick, injured or infested fish. Likewise, maintaining a lower salinity (i.e., 1.021) in an aquarium housing only fish can help suppress parasitic outbreaks.
Non-fish reef animals, such as corals and other invertebrates, must never be exposed to hyposalinity, and many reef keepers keep their reef tanks at 1.024 or 1.025.