CO2 Tank Lifetime

I have a question that has been troubling me lately, I wonder if I can get
some help from you guys.  How long is a lifetime of a 20lbs CO2 tank?

Here's my specs:
135G heavily planted
3 MH lights
Substrate Heating
Ph controller

My problem has been that I had just replaced my tank not even a month ago,
and now it has run out again.  I don't think this is normal for my size of
tank?  My regulator and solenoid seem to be working fine.  I tried to check
to see if there was any leaks (soapy water test) but there doesnt seem to be
any. I'm stumped.  

Also, does anyone have any experiences or input with the Hamilton
Technologies CO2 system as advertised in FAMA?

Any input would be greatly appreciated.

Thanks in advance.
 D C  Santiago               |        Internet: dennis at nhb_com
      Toronto, CANADA          |