Bob, three gold-plated wires would do as a simple SPST switch. If
#2 is 1" longer than #1 and #3 is 1" longer than #2 you have a switch
with #2 as the common. The end of #1 is at the high water mark. If
level falls below #2, then #2 and #3 are not shorted and the pump
turns on. When water shorts #1 to #2 it turns off

You could make them very fine, even colour them the same as the
fountain so they'd not be too obvious

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