Well, let's shitloads of different types of "float valves".
The basics::: There's going to be a line/level, where the 'float' will go from OFF all the way to ON.

You need to place that line/level exactly where you want it. Where do you want it? Depends on your setup/system. The float valve will mantain the fluid levels up to the line/level you designated. I think it'd be best to put your float valve on the more or less same height as your grow buckets, but have your res higher (3 or 4 feet higher)... so it gets the aid of gravity.

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-turtle420