On one boat we have an older Quicksilver 6 gallon portable tank with the integrated bayonet female fitting. Filled it up and very quickly noticed it was leaking between the fitting body and fuel tank, so I ordered a new gasket for, which was pretty inexpensive. However, I've now found that it also leaks directly from the fitting itself (past the check ball and seal). That fitting body is roughly $40 from what I can find, so it is almost cheaper to just buy a new tank. Does anyone know if it is possible to just change the o-rings inside the fitting with new ones?
Possible- yes. I would just replace the fitting, though.
Tank replacements aren't as easy as one might think- new tanks have new EPA mandates that went into effect in 2011 (pressurized). This requires a Fuel Demand Valve be added.