This boat I just bought has been sitting up for about a year. It has a 118 gallon fuel tank. I had assumed it would be about empty, and was planning on simply removing the old gas and putting in fresh. Well, lo and behold, I removed the sender and it is full to the brim! About a hundred gallons of gasoline. Is there a way to tell for sure if the gasoline is good? Is there an additive that can be added? Should I remove it "just in case?" If so, how do I go about it? Is this something marinas often do?