How to automatic mixing water

[Lampshade]

The issue with drums is finding a food safe drum so that the Bearings aren't exposed to the salt, and also made of stainless. Anything else would be pretty scary to use on the tank :S.

My opinion would be a simple hopper with a valve. You need something that will seal off the salt when not in use, if it's in the same area as mixing water, the humidity will make it a salt block in no time. If you made a desenct sized hopper with a small outlet, even a 0.5" opening you could open/close that to add salt. Program it so that the hopper is open for a short period then closed for 20+ seconds to allow the salt to mix. Make it cycle whilever your salinity is low.