I bought what was sold as CO2 tubing, no idea what it's made of though, sort of grey looking and quite thick walled compared by normal airline.

Lateral thinking kicking in, on the loss rate of PVC if you coil 20 foot of tubing inside your tank that would make 100% loss...........All of the CO2 being diffused. Difficult to hide though, how about undergravel CO2?

Ian