I am well aware that there is an optical sensor on the osmolator ( for the primary). Several folks in my local reef club have them. I just don't see why they should cost 200 dollars. The tunze name on them is my answer. The optical sensors do need replacement from time to time. They are about 50 dollars. Now float switches if they go out, are much cheaper. I got my reef fanatic for a trade for a coral or something, and it came with an extra switch that I still have yet to need. I set my switches in the last chamber of my sump to be about 1/8" of an inch as well. I could set them closer, but there is no need to. If I had a 2/3 part, I would need to adjust a lot since I have so many SPS and clams. Guys in my club that don't autodose with peristaltic pumps are always complaining about dosing, and adjusting their dosings. Silent is a term of perception. There are a few bubbles, but my skimmer makes more noise than my return to my sump.