You need at least one float switch and EITHER an SL1 or SL2 to use the RKL as an ATO. SL1 can monitor pH and ORP while the SL2 can monitor pH and Salinity (if you can get a salinity probe) when you use the necessary probes.
I use an SL1 with dual Digital Aquatics Float Switches. Float switch 1 triggers the ATO pump (aqualifter) when it is in the closed position and shuts it off when it is in the open position. Float switch 2 is more of a failsafe. It is positioned higher than switch 1 and only allows the pump to run when it is in the closed position.
The way I have it set up the water line in my return section ever fluctuates. more than .25"
I said Salinity probe. The SL2 will only monitor salinity with the DA salinity (conductivity) probe. They have been very hard to come by which is the reason I have not traded out my SL1 for an SL2. At first there was a quality issue. Then once they fixed the issue they replaced the first batch. After that there was a supply chain issue.
A salinity monitor would be another fail safe in the chain for the ATO. You could set the RKL to only allow the water to top off within a set specific gravity range. (1.025-1.027).