A constant volume valve provided with body, plunger and controller. The body has inlet port, outlet port, and a chamber for pressurized fluid to fill. The inlet port is connected to pressurized fluid source. The plunger is inside the body and is shorter than the distance between the inlet and outlet ports. The two end surfaces of the plunger function as two poppet valves. The controller moves the plunger in a linear back-and-forth motion. The plunger blocks the inlet port and opens outlet port when it moves to one end, and it blocks the outlet port and opens the inlet port when it moves to the other. The pressurized fluid fills inside the body when the inlet port is opened, then it expands and overflows when the outlet port is opened. The invention is particularly useful in a CO.sub.2 regulator for the planted aquarium.