A timely question for everyone up north.
Diesel does age and API studies indicate is does begin to degrade after 6 months. Add to that the environment in which it is stored, the possibility of condensation albeit low and the lack of maintenance of the O ring gasket on the cap and you can have growth in the tank.
The only way to be certain of the condition is to sample the fuel near the tank bottom and have it tested. It's usually a simple and cheap way to determine quality particularly for anyone living in a large metropolitan area like Charleston.
Call any one of the bulk storage facilities around the harbor and ask if they offer the service or if they can refer you to a company they use.
adding to what Don S/V Illusion wrote, the rate that diesel fuel degrades is in direct relation to how it is stored.
any tank that is not topped off will "breathe" thru the vent hole as the air pressure and temperature changes throughout the day... and this "breathing carries moisture into the tank to condensate as it does... this will not only allow the fuel to degrade faster, but will cause it to begin a lot faster.... the more air space in the tank, the more volume of air and moisture will be carried with each breath... a tank that is full has very little airspace in it and so very little exchange takes place...
as for the algae that grows in diesel fuel, it can become a problem if steps arent taken to prevent it before it starts, and as big of a problem as it can become, its cheap insurance to use an additive and not allow it to live in the system to begin with.. and it is usually, but not always introduced into the system by adding fuel to the tank from a contaminated container or source....
the rate that diesel degrades is usually not an issue if it has the proper additives and filters (within a couple of years).... there are all sorts of diesel engines in trucks and equipment around the globe that have been sitting for many years without running, and providing they were running good at the time of storage, they will start up and run just fine many months or years later... and the only thing that is really different in the systems, is the atmosphere... the marine atmosphere has more moisture in it than does the air 5 miles or more inland...
so as long as your tanks are "breathing" the marine air, it is going to degrade your fuel faster... with good additives, at 18-20 months i wouldnt worry too much, but after that i would think about testing it, (it may still be perfectly good) or taking a cruise and burning some off and freshening the tank with new...
