First thing I would ask you is....what does your fuel filter
look like? Is it black or any black looking snotty stuff on it? Have you had any fuel problems yet? The fact that your tanks
were full is a good thing. The biological contamination needs air and water to grow and having full tanks makes it impossible for the living organism to grow. However if they were not completely full there is a possibility for contamination to exist.
Maybe your fuel was treated with a fuel additive, perhaps a biocide that kills the bacteria and prevents it's growth. With a twenty gallon tank, I would not be too concerned unless your fuel filter
To avoid spending money
on a fuel polishing bussiness (such as myself) and being that you have a small tank. I would recommend pumping your fuel into 5 gallon containers if your concerned about it and physically cleaning
your tank if you have good access to it. You can also dip your tank with water finding paste and it will tell you the level of water you possibly may have in your tank. Add your own inspection
plate with one of these...Seabuilt - Access Plate Systems
For the most part, if your fuel filters look clean and you don't have any water in them, and you have not been shut down at sea because of dirty fuel that get sloshed around and breaks it loose then don't worry about it.
I would recommend that you carry extra filters in the event that your engine shuts down (usually in rough water) due to contamination that breaks free from the tank when sea conditions are rough.