As you know, evaporated water needs to be replaced with fresh water as salts do not evaporate.
I have seen many people use electric float valves directly in the sump that activate a water pump when the water goes below a certain point. In my opinion this is a disaster waiting to happen unless there are serveral safties built in, this can be costly. If the float valve or pump fails OPEN not only will you flood your floor you will also most likely kill all your livestock because the salinity levels will drop radically.
I have been using a gravity feed with float set-up for the past 4 years without ANY issues. If kept clean (clean algae and crusations off the sump float valve) it will never over fill your sump with fresh water. Here is a list of what you will need to “DIY” this set up, an RO / DI unit (for both top-ups and water changes) 15 gallon reservoir, Auto Shut Off pressure valve by John Guest, 1/”4 Solenoid to turn main water to RO on / off, electronic timer to control the on/off solenoid, two Plastic Float Valves (these are GREAT, more on this later) 25 feet of ¼ line (assuming your water in and bad water out are far from where your Ro unit is sitting) 1/4″ bulk head to connect the 1/4 tube from the res. to the sump valve.
How does it all work? It’s simple actually.
In order to keep my reservoir filled, my RO-DI runs for about 1 hour per day (electric solenoid). I do not get much evap so that is more than I need. It fills up the top-off reservoir until the float valve blocks the RO out hose causing backpressure, this pressure is what activates the auto shut-off , thus turning off the RO unit.
I have the second plastic float valve drilled into my sump, the other end ties into my reservoir. The best part about this set up is that it is gravity fed, NO pumps. The valves work in such a way that they open ever so slightly as the water evaporates. Therefore the water trickles in at the same rate it evaporates. Salinity is rock steady!
Notice double fail safe? There are skeptics but I love this set-up. No extra pumps and it’s safe AND cheap
Notice the second barrel? Once my system is established one of the barrels will we used to hold fresh RO water and the other will still be used for this auto top-off set-up BUT it will top-off with Kalkwasser (limewater) to does Calcium and Alkialinity.
Here is a pic of the resevoirs I’m using. They are food grade 15 gallon barrels made for alcohol beverage fermentation.