This bogus information seems to be perpetuated by the pet shop trade. The exact source of the myth is unknown, but it seems to have started in the early 1980s. I have been told it may have originated in an article by Herbert Axelrod, but cannot confirm. No matter, the concept that keeping a marine aquarium at lowered salinity, with specific gravity of ~1.019, is commonly cited as helping to prevent disease outbreaks in saltwater fish.
This idea has never been proven to help fish in any way. A specific gravity of 1.019 is far short of the level of 1.009 needed to combat ich during hyposalinity treatment. The practice may save some money for the fish supplier in it requires less salt, but it does not prevent disease. Although it doesn’t seem to hurt fish, lowered specific gravity is not good for corals and other invertebrates. They need to be maintained in the range of 1.025-1.026 for long-term health and well being. The widespread use of low salinity husbandry is why one needs to carefully acclimate new fish. If the fish is maintained at low salinity at the fish store, a sudden change in salinity can be damaging to the fish.

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