I'm embarrased to say I went through 3 TANKS! before I found my leaks.
Mine leaked at the tank/regulator connection, and at the needle valve.
soapy water, didn't work,
leak detector fluid, didn't work,
Finally, I submerged the entire system under water to find the leaks (the old bicycle tube approach, for use old timers out there). Had to do it quickly, as to avoid ruining the regulator (regulator gauges fill right up with water when submerged, I think they can be ruined easily this way). Then I used a combination of teflon tape at the regulator/tank, and massive globbs of silicone at the needle valve connections. Let it all dry for 3 days before hooking back up. It worked for me, been going almost 3 months with current tank, no drop in pressure except temperature fluctuations, as someone mentioned above.
As an aside, my house plants all ate up the co2 in the house during the leaks, you should have seen the new growth!