i made an automatic top off system using a mercury switch to a relay that activate a small powerhead that sits in a bucket of ro water . I used it for 5 years without any issues then got tired of filling the bucket with water so.
On my new tank i redesign the mercury switch to activate a sprinkler head solenoid that i got from Home Depot to turn on the city water which then runs thru a ro/di/ then thru a DIY kalk reactorfrom there thru the walls and to the tank. been working for 5 years no problems. This is one of the best things i've ever made because i can go on vacation and never worry about evaporation of the tank and the stabality of the water is unbelieveable.
The beauty of the mercury switch is that it's nonmechanical and therefore cannot fail from jamming or never needs cleaning. it's easy to adjust too.
here's the switch in my sump that i made using a spice bottle and a magnet to adjust for water height.
going to the sprinkler solenoid that sits in the garage so i have no junk in the house.
then to my RO/DI that also is in the garage. You can also see the hole in the dry wall that has the water hose and control wires to the floating mercury switch in the sump..
from there to my DIY kalk reactor that automatically adds calcium and magnesium. This is also in the garage.
and finally to the back of the tank. The refill hose and control wires that then goes to the floating mercury switch in the sump under the stand.
Now i never have to topoff evaporated water. I also put a switch on the solenoid so i can make fresh RO/DI water for my salt mix.