OK, here comes Miss or is it Mrs. Buttinsky. I have been preaching about floats for a long time but haven't had reason to get on the soapbox for a while.
Coming from someone with experience. DUH??? I put a piece of filter media in a wire mesh basket in the entrance to the spillway from the Skippy. The media got filled with crud and blocked the spillway====results, Skippy overflows. Luckily Dh is leaving for work and discovers it before I had any loss, just a little water. Didn't learn my lesson and did it again. Another lucky save as I discovered it this time. Now if I want to filter out the algae quickly, I put the filter media in mesh baskets and sit them under the drops of my waterfall. So if they get crudded up, it just runs around and over top of it and stays within the waterfall.
Next experience---a plastic shopping bag blows in the pond and gets sucked across the opening of the skimmer which lowers the water in the skimmer and the pump runs and runs and runs and gets so hot I am shocked it didn't burn up. Soooooo, I purchased a pump with a built in float. The float drops down, and automatically shuts off the electricity supply. A few years later when that one dies, the place I had purchased that brand from, said they didn't have any with a float and wasn't sure that brand even came with a float any more. So I went on Smith and Foster website
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=5344&rel=1 It was only about $45 and I zip tied it inside my skimmer tank and now I no longer have to think about the water getting pumped out of my pond in any way. To me it is worth what I spent when I know it protects my pump, my fish and my water plants.
Which reminds me, never put your pump on the bottom of your pond for that reason. If you do pump it down, don't make it so it's going to take every drop of water you have.
I'm lucky as we are not on city water.