Pull a sample and see how it looks. Like I said with mine, the fuel is 14 years old and it looks nasty but it fires right up. Don't know how long it would run at speed before the filters plugged up. I don't know if the fuel is dark from bacteria or it's just dark because.

50 gallons isn't a lot of fuel and as long as there isn't a bunch of water in it, you shouldn't have a bacteria issue.

Kevin