Neat thread... I wish I had seen it earlier, so I wouldn't have had to "re-invent the wheel" earlier this week, while designing my own system. Funny thing is, I used the very same solenoid!
The biggest difference for me, is that my system will be externally housed in a small project box, with 2 indicator lights and a SPDT toggle switch for quick shutoff when working in the tank. As an additional precaution, I have added a panel mount fuse.
The reason for the SPDT switch & green indicator light, is so that I will always know when there is a constant supply of power. The 2nd circuit will be controlled by the HI/LO float switches, and they will have an amber indicator light, to show when the circuit is complete and the solenoid is open and feeding water into my kalk reactor.
Initially, I was going to use a peristaltic pump, but got to thinking a bit more, and came up with this idea instead, as it has better failsafes.
Here's my complete unit. The water INLET is on the right, and the OUTLET is on the left...
In my diagram below, it will be hooked into the system where the DOSING PUMP is currently shown...