If I may (not to steal Colin's thunder...)
Need to raise SG--add saltwater.
Need to lower SG--add freshwater and, if necessary, take saltwater out of tank
Your hydrometer's measuring Specific Gravity, which is basically the same as salinity (technically it's not but I won't launch into THAT...besides, I don't think I understand the differences fully myself, either!
) In other words, it indirectly tells you how much of your tank's water is dissolved salts. Higher means more salts per milliliter (or gallon, or however you want to measure it) lower means less salts. So, to raise the SG, you need to add salt to the tank. Not directly! You need to completely dissolve it in some water in a bucket beforehand. Generally, you add saltwater to the tank at the same concentration you want the tank to be. If you need to boost the SG, you just add saltwater instead of freshwater when you top off the tank. As water evaporates, it leaves salt behind, so this method will gradually increase the tank's total salt content.
By the same token, if you need to lower SG, you can add just straight RO freshwater. That will dilute the tank and lower SG, but it won't actually take salt OUT of the tank. If you have a too-high SG even when the tank is full, then you need to remove some saltwater and replace it with freshwater. Generally, I try to keep my tank's salt levels constant, replacing only as much as I take out, and adjust the SG as necessary with freshwater.