Don't you just love a bit of controversy

The CO2 duration calculation is completely independant of tank conditions - it takes no account of how many bubbles per second are required to acheive a certain concentration in a certain sized tank but rather how long a certain amount of liquid CO2 will last when released as a stream of certain sized bubbles. The concept is sound

- same as Â£100 will last for 100 days if spent at Â£1 per day or 10p per hour for 10 hours per day but the calc is quite sensitive to bubble size. So for instance 2kg CO2 run 24/7 at 2bps should last: 4.5mm bubble - 132 days, 5.0mm - 96 days, 5.5mm - 72 days, 6.0mm - 55 days BUT you will need over twice as many 4.5mm sized bubbles per second than 6mm bubbles to deliver the same amount of gas and this would be reflected by ph and plant growth in your tank.

All I can say is that it works for me

- I did the maths to try and predict how long my 3kg cylinder would last when I first got it in January - so far it's still going strong at about 3bps for 10 hrs per day. The calculation estimates a cylinder life of approx 7-8 months with a bubble size of 5mm so I expect that it should run out soon

a 2kg cylinder run under similar conditions should last approx 5 months and a 500g cylinder 5-6 weeks. On the whole the results seem to match other peoples experiences but there are always exceptions. Perhaps I could test it better if we could compile a list of cylinder life against co2 weight, bubble rate and on-time. Any takers ?

Charlie