I believe there are also devices which will automatically top up your tank with water.
I accidently have pump protection in my system without building it in on purpose. My water pump filled the fish tank too fast so I put a "tee" in the line and piped another line back down to the sump tank (with a valve of course to control the water amount I want to go back to the sump and the water amount I want to go all the way up to the fish tank). Right now about 1/2 or less of the water goes up to the tank.
Anyway, one day the water in sump tank was low and no water was going into the fish tank and I panicked. I looked down in the sump thinking that my pump probably burned up...but was pleasantly suprised.
As the water got lower in the sump tank, the pump would pump all that it could up the pipe, but because the water level was so low it couldn't pump any further than the horizontal pipe I had installed ... so the water took the course of least resistance and went down the pipeback into the sump again and kept doing that for hours. So the pump didn't burn up and didn't run completely out of water.
This was all an accident...as much as it kills me to admit it....but maybe you can apply the principle in your system ?? Maybe not :-)