Tri Clamp Check Valve

A check valve allows liquid to flow forward, but not backward. As long as you're moving fluid at > 2 psi, the valve will open and allow liquid to flow freely forward. When pressure stops, an internal spring closes the valve automatically preventing liquid from flowing backward.

Maybe you have an open impeller pump connected between your tanks. Unless you use a check valve, pressure will push liquid backward to the source. You'll also commonly see check valves used at the outlet of a lenticular filter where you don't want to risk back pressure blowing through the lenticular media and damaging it.