I like Tesla's idea best; photo optics is a good option, or use a pressure transmitter or pressure switch. Even though you're talking about 6P submersible enclosures, keeping electricity out of water is always a good idea. I'm all too familiar with reed switches. They fail too easily; moving parts. I try to design fountains as simple as possible. Whenever electronic water level controls are necessary, conductivity sensors are the simplest to maintain, modify and understand.
