Moved into a new home a few months ago with a well and water softening system. There's a brine tank, and what appears to be two softening tanks both with timers on top. One of the tanks has a line labeled "add salt" and when I put a flashlight to it, I can tell that it's low.

Do I add salt to the brine tank, to the softening tank, or both? And how high?

Thanks in advance,