This is a small check valve that can be used to control the direction of flow in standard aquarium tubing (4mm inner and 6mm outer diameter) and requires a 4.5mm BB pellet. As in the cut-out picture of the valve, the main chamber contains a pellet. If the flow comes from above, the pellet rests on a raster and fluid can flow around the pellet. If the flow comes from below, the pellet blocks the tubing on top, going out of the valve. The flow can only go from top to bottom. The direction of flow is indicated on the outside of the valve.Print the object at full infill without support. You can either choose to print it flat or standing up. There's a few shallow bridges in both versions, but both are printable. I prefer the flat model. After printing, insert a BB pellet into the valve. Fit tubes (cut of at a right angle) on both sides and glue them (e.g., with ABS juice) to make an air/water tight connection. If the pellet does not block the tube, make sure the hole on the lockable tube is at least 5mm by drilling it out a bit.