A little water accumulates in any enclosed tank if air can enter and there are cold/warm cycles during the day. We have to check for water in the bottom of airplane fuel tanks each time we pre=flight. We simply drop a small stream out and that takes care of it. We use alcohol in gasoline tanks to absorb the condensate. Since a lot of gas nowadays comes with 10% alcohol added, that does it. A diesel tank will accumulate condensate just like anything else. In nice warm climates the temps stay high enough to limit the condensation somewhat but in the spring and fall there are days sufficiently above and below the dew point to deposit liquid water into the gas tank. Keeping a full tank is a way to reduce that but -------.A can of "Heat" in the tank now and then will do it.
Alge will usually grow in diesel fuel stored for a time --as in a car that is not run. Warmth is very good also. I have some diesel in a boat tank which hasn't been touched since 1989 and I run it a little now and then to keep the old girl lubed. No trouble so far. Cars that run through fuel on a regular basis do not usually suffer the "Attack of the Killer Alge" Steve