These cut off switches sense low water conditions and will automatically turn your pump off to prevent damage. Convert any manual pump, up through 1/2 HP (10 max. run amps), to automatic.
Summer usually means increased pond water loss due to evaporation, which of course leads to topping off our ponds more. You should always add dechlorinator when topping off the water level of a pond. Any amount of chlorine (such as that found in treated water) can be harmful to your fish. Even if it is not enough chlorine to mean immediate death of the fish, this chlorine can still cause harm to proper gill function.