What is the correct specific gravity of water in a reef tank?
Before outlining the correct specific gravity of water for a reef tank, it should be noted that we should be aiming for the correct salinity and the specific gravity is simply the method chosen to estimate the salinity. So the first question should be, What is the correct salinity for a reef tank? While there are variations in salinity of surface seawater from ocean to ocean (Atlantic is more saline than Pacific) and from location to location, the salinity is generally fairly constant, especially in reef areas. It can be assumed that the salinity of reef water is very close to S=35 and reef aquariums should be kept as close as possible to this value. There is absolutely no reason to keep a reef tank at a salinity anything other than the natural salinity for the reefs from which the organisms were collected. Far too many hobby books recommend salinities (and specific gravities) that are significantly lower than natural reefs and I have yet to see any present a valid reason to do this. W
