I have had the standard 3 gallon tank that came with my Mercury 9.9 HP and it has held up well (hasn't leaked), but it is more round than when I first bought it due to expansion. You would probably want a 6 gallon tank. Maybe leaving the manual valve open all the time might help, but it could be a one-way valve, and your fuel would evaporate over time. My tank doesn't loose fuel, even after almost a year of non-use. But it is annoying that it isn't as flat as it once was.