The garden pumps are activated based on water levels, which combined with timers ensures the water should never overflow or underflow. Preventing underflow is important for this ebb-and-flow design, because if the pump sucks in any air the siphon effect breaks, and the grow bed will never drain.
Here's my current water level sensor design, basically Kapton/Mylar tape and metal foil:
Unfortunately it looks like my water level sensor design needs rethinking - they have stopped measuring the water level properly and now behave as a boolean (ie, there either is water or there isn't).
My current theory is that they have accumulated a layer of nutrient salt, making the whole length of the sensor slightly conductive, so it looks like the water level is higher than it really is.
Oh well, back to the drawing board!