ISTA CO2 Bubble Counter + Check Valve - aquarium tank water plants 4/6mm tube Item is easy to install. The hose mechanism is quite simple (similar to canister filters), its perfect to use with the MAX MIX CO2 REACTOR, because it allows to control the CO2 flow, once its impossible to control the CO2 flow using only the reactor. Two functions as CO2 bubble counter & check valve in one unit. Apply to both internal or external use.