DIY Top Off

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Can someone explain the float valve ie. I get the idea that when the water level drops below a certain level the valve opens allowing water to siphon in from the reservoir, but how is the float valve connected to the line from the reservoir. I am in a similar situation, limited space next to the sump for a reservoir, I thought I'd have to go with float switches and a powerhead/pump but if a float valve and siphon is easier then I'll go for it. Also anyone running kalk topoffs with a float valve?