I had a Thread on it, but with the Forum format change- I can't find it, but, I can explain it very easily. Bascially, it only used Gravity as it's power. It all starts with my RO/DI- mines under my kitchen sink b/c the RO goes to a 3g storage tank- to a drinking tap on the sink, and to my icemaker. The DI goes thru my basement ceiling area and over to my basement sump area. - at that point there is a John guest plastic valve for on/off purposes- I usuallyu leave it partialy on- - this goes to a clear plastic container that is above the level of my sump, and has a nice, plasice, heavy duty float valve with a fairly large float on it. When it reaches the level in the plastic ocntainer I have set on the float, it shuts off the RO/DI input automatically. - from there, the line goes to a float valve I have in my sump, the same all plastic, heavy duty valve. - this is also set where I want to keep my sump level at. Doing it this way, any little bvit that evaporates, a little bit of pure RO/DI water is put back. My SG Stays Rock steady and has for years- very important for higher end corals, etc. The valves are above the water line obviously, and the plastic float is partially in the water.- It is VERY bouyant, it's always going to shut off the flow of water as the water rises- always does, always will. The whole thing is a low pressure system, needs no siphons, no pumps, no sensors, no electric power, no relays, etc., etc. The whole thing is less than the $30, and that includes 2 x heavy duty all plastic float valves I got off Ebay for about $10 each shipped. Here are some pics of parts of it; look closely and you'll see the the container/resevoir above and behind the sump. You can see the 1/4" line going to the float valve too I think. It's coverd and does have a vent though. Here's a pic showing the final/sump float valve. In order to use it I opted for this kind of set up fastened to the plastic strainer basket shown. In this photo, I actually had the JG valve off, I think I had just done a WC, and usually shut it off so I don't add some RO/DI prematurely when the water level is temporarily down in the sump suring the WC. Works awesome, never fails, and I don't think it will, it's too simple!