Topping up the fuel tank for winter

[ffiill]

I have three large plastic tanks for red, white and heating oil. The tanks are about 1200l each, and stand outside. I know from direct experience that water gets inside each tank (there are traps on the outflow line). The only way in is from atmospheric humidity.

Peering in is not an option for most people with tanks on board. Prevention is key; response to a water initiated bug or degradation is expensive, and can be easily mitigated by standard over-winter procedures, e.g. filling, and/or isolating to prevent water vapour getting into the tank.

My experience is quite the opposite so it must depend on the prevalent atmosphere where you are.
My two 70 gallon steel tanks on the boat which has spent the last ten years in nw highlands has not collected a drop of water in the fuel filters.
Similarly my steel 1000 litre diesel tank and my 1400 litre plastic keroscene tank show no water inside.
My plastic central heating tank can be seen right to the bottom from the top inspection hatch and has a simple water trap filter on its bottom outlet.
Similarly my diesel tank has a water trap and fine filter on its outlet and I havnt had to touch it in 5 years.
I often have part empty tanks although our winter temperatures are consistently cold and it doesnt vary much as we loose the sun most of the day from now until the end of Feb.
My biggest problem is dead leaves and Daddy longlegs getting in the tanks!