[Przemek_PacSun]

Solenoid valve is controlled by CalcFeeder controller.
About opticals sensor - there is no any special issue with that, usually(depending from flow/media type) every 6-12 months you should clean it if controller can't read proper water level inside mixing chamber.
For example - in my home reactor I never cleaned it(since two years). We had one or two reports where Customers used media wich started dissolving in higher pH and media "melted" - it was clogging optical sensor , but any typical media like CarbiSea ARM, natural crushed corals, Knopp Koralith, AM Hydrocarbonat work great...