By this CO2 check valve between the CO2 bubble counter and the CO2 system will prevent water from the aquarium into the CO2 regulator or CO2 valve.
The check valve can only have 1 direction to move. If the pressure of the other side on the non-return valve is larger, this will block. Water can in this way never out of the aquarium siphon and cause damage to the CO2 solenoid valve or the CO2 pressure regulator.
On both ends of the CO2 non-return valve is easily the CO2 hose to fit. Note when connecting to the CO2 valve on in the right direction. Advisable is to have the valve replaced annually to ensure a safe operation to guarantee.