If you dont mind using a stepper why not have a feed tank of water slightly lower than the build tank with a hose connecting to two. Then use a stepper to lower some displacing mass into the feed tank, the level of the saline in the build tank will also rise.

In-fact you don't even need a stepper, a roll of steel beads on a large reel being slowly unwound by a slow motor into the feed tank would slowly increase the saline level in both tanks