An alternative for electronical level controlls is the floating valve. A floating valve controls the water level very exactly. It is important that the water reservoir is above the floating valves so that the water runs down. With one reservoir you can supply many tanks. You only need one T-fitting for the water tube and one floating valve for each tank.
If the water level is falling the valve opens. Fresh water lows into the tank until the normal water level is reached. The floating valves are for fresh and sea water. No salt crusts will occur. The full-plastic version is for corrosive media, too.
The floating valves are useable with max. 4 bar. It is very important that the used water is free of particles. Otherwise you need a small sediment filter before the floating valve. To mount the valves you need a hole in the tank or tank lid. The floating valves has a 1/8" female connection.
Important: do not connect a R.O. unit directly with the floating valve. It is only usable with a pressure tank or a pressure-less tank between R.O. system and floating valve!