Reef Specific Gravity.

[ghztew]

Quote from Reef Sources Guid:

"It is not the specific gravity that is so important, but that you consistently maintain your water at whichever level you adopt. So, if your hydrometer is off by .002, but you consistently maintain your salinity at 1.024, your animals will be fine. If it wanders all over the board, the osmotic changes will eventually stress your livestock, particularly the invertebrates. "

http://reefsources.itgo.com/guide/water.html