If you have a way to heat the water you'd get more of the gas coming out of solution. I would think with all the fracked
wells in the US that your situation is not all that rare and lots of people could be doing this.
[Edit to add some additional thoughts]
You might want to try this without the fan. Methane is lighter than air (density at 25 C is 0.65 g/l, plain air is 1.2 g/l) so if the containment vessel is allowed to stratify the methane will collect at the top. A small pump could then harvest the methane and move it to your storage tank.