I recall a few weeks back someone ( sorry, I forget who, ) suggested 
       using yeast & sugar to produce CO2 in a small jar or similar inside 
       the terrarium.  I am just in the process of doing this myself - I 
       figure it might be a good idea to stop the neck of the jar with 
       cotton wool or suchlike,  just in case escaped yeast might cause a 
       problem in the terrarium,  or use an external jar connected by tubing.
       It's certainly a lot cheaper than cylinders...
       
       Just my 2 pennorth...
               Happy growing,
                       Peter
  Peter Cole   | carnivor@bunyip.demon.co.uk | snowing...
Swansea, WALES |                             |