Just as hot air is lighter than cold air. Hot water is lighter than cold water thus changing the SG
. As I said earlier a hydrometer only checks the weight of the water in relation to itself. It then has the appropriate conversion marked on the side for salinity. With a hydrometer that is made specifically for aquarium use the manufacturer has taken "normal" tank temperature into account. Some of the better hydrometers will have the standard that was used to calculate the salinity marked on the packaging. Even though the water is lighter or heavier, whichever it may be the salt content has not changed.