I'm about to play this game too.
My current plan was to dredge up the thread in here where someone could get plastic float switches for about $20ish... and go from there.
Plan being to have a container of freshwater, and have the floatswitch control an airpump pushing into that container, forcing freshwater into the tank.
You could also have the switch power a pump if you wished.
I would like to know if there are any alternatives to the float switch however... like perhaps setting up a probe or something similar in the tank to sense the water level. (yes, I'm paranoid about flooding the tank )