A "full" tank really isn't full. It has 5 pounds of liquid CO2 in it, with the rest of the volume filled with CO2 gas, and the pressure of that gas depends on the temperature. At room temperature it is around 800 psi - those gages aren't terribly accurate, so just look for about 800 psi. When it is cold, right after being filled the pressure will be lower.
It is hard to overemphasize just how tiny a leak is involved when the tank doesn't last long enough. A two bubble per second rate, which is comparable to what we run into the tank is not easy to see with soap solution, unless you let bubbles accumulate, but that would reduce the life of the tank by half. A much lower leak will deplete the tank in a few months, even if none goes to the aquarium. It isn't just "hissing" noises that are the leak signs to be searched for. When I first set up my CO2 system I was amazed by just how small the leaks I finally detected were, and one time my 5 pound tank was empty after only one month!
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hoppy
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