Water is the problem with heating oil, or diesel fuel or jet fuel... There is a microbe that grows on the film between water and kerosene [diesel, jet, and heating oil] . The problem is much worse in warm weather, but you can still get some microbial growth in cool weather.
There are several products that will reduce or prevent the growth of the microbe... one of which is gasoline.. but I certainly don't recommend adding gasoline to your fuel oil tank..
The best that I can think of is the frequently drain the sump in the tank, to remove water that WILL accumulate from condensation on the inside of the tank walls.
You might ask your heating oil supplier... if they will talk to you still..
to see what they recommend..