uxcell Aquarium Double Head Water Airline Check Valves Connector 12 Pcs Red Clear Material : Plastic;Color :Clear, Red;Package Content:12 Pcs x Aquarium Check Valve Total Size : 5.3 x 1.5cm/2.1" x 0.6"(L*Max.D);Hole Diameter : 2mm/ 0.08";Fit Tube Inner Dia: 0.6cm/0.2" Using for fish tank, fishbowl and aquarium. Protect you air pump by preventing aquarium water from back siphoning during a power outage. Great check valves help you creat a healthy aquarium environment.