Water flow through plumbing can be controlled with a couple of types of valves.
The ball valve is a simple device that turns 90 degrees to open or shut water flow. It can also be left open in between to regulate water flow. They can be used to shut off water to devices that need to be removed and repaired such as water pumps.
A gate valve is a device that allows more precise control of water flow. As it opens or closes, a gate raises or lowers inside to control flow. These are used to regulate the amount of water flow through devices such as chillers or to slow down the force of the water going into the aquarium.
Ball valves can become hard to open or close over time, so install them with union fittings so they can be removed and replaced if necessary.
