Knife Valves allow you to tightly close off the water lines of the connected piping to allow for maintenance on joined mechanisms such as pumps or filters without shutting down your pond.

Gate valves (also known as knife valves or slide valves) are linear motion valves in which a flat closure element slides into the flow stream to provide shut-off. Gate valves and knife valves are designed to minimize pressure drop across the valve in the fully opened position and stop the flow of fluid completely. 

The direction of fluid flow does not change, and the diameter through which the process fluid passes is essentially equal to that of the pipe. Hence, they tend to have minimal pressure drop when opened fully.