Before reef tanks became doable, people who kept FO tanks kept their tanks at 1.021~1.022 in the mistaken belief that this helped lower the stress in their fish. Well, this is just plain silly as fish are osmoregulators. Osmoregulation - Wikipedia, the free encyclopedia They REGULATE the amount of salt in their bodies. That's how saltwater fish can swim up freshwater streams to spawn and their babies can leave a freshwater environment and adjust to saltwater. What's the problem with staying down at 1.021~1.022 you ask. Plenty. Our inverts (including corals) are osmoconformers and have no ability to adjust to surrounding saltwater salinity. They can live that low but they don't live nearly as long. Salt is cheaper than constantly replacing urchins, crabs, snails, cucumbers, and other clean-up-crew members. That's why for reef tanks (or FO tanks with a clean-up crew) keep their tanks closer to average NSW values. The reality is that the animals that live in our tanks are a whole lot tougher than us hobbyists give them credit for. They don't have constant salinity, temperature, lighting, or pH. I still recommend that we try to maintain a constant balance in our tanks just to remove one or more variables but in the wild they don't get that. A tropical depression might form, turn into a tropical storm, then turn into a hurricane. While this is happening, a coral might not get hardly any light at all for a week or two. Not only to add insult to injury, there might be tons of rain dropping SG from 1.026 to 1.018~1.019 temporarily. Think about the corals that live in tidepools on patch reefs. Everyday, the tide goes out, the water they are living in starts evaporating, temperatures and salinity rises substantially and pH drops, then a couple hours later the tide comes back in with cool, less saline, higher pH water and they live through this day in day out. Think about what happens when an upwelling from the abyssal plains hits a reef. Temperatures drop 15~20 degrees, pH drops, salinity swings are outrageous. Thought you might find this video interesting. Mind you, these corals are all alive just waiting for the tide to come back in.