jimd-nice device, so, looks like a PVC flange(4"?) a length of pipe(how long) and a coupler(4"-2"?) with some sort of cap, so you can add kalk, and keep out air. is the blue thing just your top-off pump? i guess the mixer pump is on a timer? how do you prevent it from dosing while it's mixing, and getting undissolved kalk in the tank? looks like an easy enough build.