"mikesb" wrote in message . ..
The problem with the non-mechanical device is the water level must drop 1"
before the switch turns on. I have the reliable Ultra-life float switch
which kicks on when the water level drops no more than an eighth of an inch.
It may be better for the aquarium environment if the fresh water is added in
much smaller amounts.

Well, depending of the volume in your pump section of your sump,
water drop 1" or 1/8" might be neglectible in terms of total
salinity of your system.
I am sure you know that salinity of surface ocean waters fluctuates
quite dynamicaly with every rain on the reef...
There is no need to keep salinity of the tank water perfectly stable.

The simple test to do would be to measure salinity of the tank just
after top off - then after water evaporates to create 1" drop in the
sump, return pump section. Compare two values and you will know
the difference is not important.