Get it out of there.
Louis: I would recommend that you pump as much of the contaminated diesel out of the tank as possible. Then you should add a couple of gallons of clean kerosene and some tank cleaner. Let this stand in the tank and hopefully swish it around as much as possible (this is assuming that you do not have some type of inspection port in the tank). If you have an inspection port you should be able to pump all the old fuel out and just clean the tank with kerosene. Do not forget to flush the fuel out of the return line too. You may even consider replacing the supply and return hose. Pump all of that out of the tank and refill with clean fuel. Replace your filters. If you have something like a Racor you will have some very good filtering. You should run your engine for a few hours (4-8 hrs, this will be about 3-4 gallons of fuel) and then change your filters again. Assuming that the system is not contaminated you should not have any problems. Be sure to add algicide with every fillup. .
