That sounds reasonable...
This is how I will do mine...
Basically... top off water is above everything.
1/4" tubing comes out of the bottom of the tank (john guest Bulkhead fitting) and then t's. One leg going to the sump for top off... it is triggered when ever the water level is low.
leg 2 goes to the kalk reactor and then the sump.
It is always on, flow reduced to a drip by a valve. if the water level raises to high, the second float valve will shut both solenoids.
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Using what you are saying... you could do it a little different...
2 bulkhead fittings. 3 solenoids. 1 t fitting. 2 valves
2 feeds from the top off, valve at each to shut off the system, then solenoid for auto control.
1 feed to the kalk reactor, one to the sump. Have the kalk t back to the main top off line, so that when the top off kicks in it will clean the kalk out put, one final solenoid as a fail safe that would close off when the high water water switch is tripped....
hummm...