EJ:
It will much depend on your consumption. In general you will use about 1 pounds of CO2 for every 1.5 pounds of media you need to dissolve. The overall reactor efficiency is about 75%. With that utilization one pound of media will provide about 5040 dKh of alkalinity to 1 gallon of system water.
So to approximate how many days a cylinder will last you can use the following formula based on the above numbers:
Days = (CylCap x 5040)/(0.67 x SysVol x AlkCon)
Where:
Days = How many days a cylinder with a given CylCap will last
CylCap = Cylinder Capacity in pounds of CO2
SysVol = Your aquarium system total water volume in gallons including sump and periferals
AlkCon = Daily Alkalinity consumption in dKh per day
as an example:
If you have a system with 130 gallons with an alkalinity consumption of 1.5 dKh per day, a 5 pound CO2 cylinder will last:
Days = (5 x 5040) / (0.67 x 130 x 1.5) = 25200 / 130.65 = 193 days
or about 6 months and 12 days
In other words and the equation (and logic) can tell:
The larger the cylinder, the smaller the system and the lower the alkalinity consumption, the longer the cylinder will last.
Note this is an approximation as with many things CO2 can leak, pass trough the reactor unused, the assumption of reactor efficiency changes with reactor design etc so consider these numbers +/- 20 % but will give you a rough order of magnitude.
Enjoy!
