FWIW...
Although I have had this too, it has never been so bad that it required multiple rags/diapers. The advice above re: the vent line routing seems useful.
Also:
RACOR makes this, designed to alleviate this exact issue:
West Marine
www.westmarine.com
(and yes, I learned bout this from Maine Sail, to quote: "I now have an H2Out vent filter and Racor Lifeguard LG100 installed to keep my fuel contents drier when the tank does have fuel in it.")
On which note, H2Out:
https://h2out.com/ ; filter mentioned is:
https://h2out.com/fuel-guard-avd-2/ )
[I do not run these, but may one of these project days. PS: I bet there is a correct order for these, say LG100 closest to the tank, then H2Out closest to the vent opening to air... that way fuel backing up will get trapped before getting into (and probably ruining the dessicant in) the H2Out.]
Aside: "Maine Sail", who ran Marine How To (
https://marinehowto.com/ , lots of great articles still up) had a nasty stroke and the business is now closed, though the articles are still up. And the articles are great (as was he, when I once interacted with him). I would encourage anyone using the site to donate via the 'donate' button... anyone who has met a stroke survivor knows how hard life suddenly has become for them, and at a time when suddenly their ability to earn income has evaporated.)
The reasons I have heard to keep the tank full (I keep mine pretty full; a dockmate tops his up after every single use using a jerry can and pump or siphon. Actually, I know a number of people that never fill at the fuel dock because it is so hard to manage not spilling and everything else, and sailboats burn to little fuel so the volumes needed for a fill can be small. So they top up / fill from jerry cans. My marina prohibits this, and I honor that, but it makes sense to me as a plan.) Why?
1. Growth interface area: The biogrowth in the tank supposedly grows at the fuel-water interface. The smaller that interface, the less area there is for that growth. Topping the tank
all the way up leaves only the tiny area of the bottom of the fuel fill line as an interface area. So less growth.
2. Smaller "tank breathing" air volume: As the tank cools at night, the air volume in it will decrease, sucking air in the vent line. That air is moist. The air gets forced out in the AM when the tank warms again, so the tank "breathes." Every time this happens -- every day -- some moisture will inevitably get left behind. That moisture is the eventual source for biogrowth. [N.b.: the H2Out filter is designed to reduce this issue. The fact that Maine Sail uses one suggests to me that he believes in this issue;
the article I see from him discusses condensation in an _empty_ tank in the winter in Maine. I suspect the amount of water in the air in a Maine winter is low -- note: not the humidity, which is the fraction of the total water vapor the air can hold at a given temperature, but the absolute vapor pressure of water which drops very fast with temperature. 50% humidity air at 60F has
a lot more water in it than 50% humidity at 33F...) -- and so maybe the issue was less evident. And certainly he ran an H2Out on his boat vent line : )
3. Less fuel sloshing to stir up sediment: When I chartered, from a very reputable place, I regularly encountered engine failure due to fuel filter clogging. This place [OCSC, for those who know it] chartered tons, but the engines never ran much because SF Bay has so much wind they were only used to get out of the marina. So the fuel got old, and stuff grew. The failure only seemed to occur on especially rough days, and the staff explained that the resultant sloching stirred up any sediment/etc in the tank, which is why filters clogged only then. If the tank is half-empty, then when it gets rough (and maybe you really want that engine at some point?) the fuel is sloshing like crazy and the fuel will have maximum suspended crud. On the other hand, if that tank is nearly full... very little sloshing and crud will get stirred up less.
So I keep my tank prety full. As always, YMMV.
PS: Biocide: I also fill my tank at the fuel dock, and that fuel has biocide pre-mixed in it. You can (and should) buy and add biocide separately if filing regular diesel (from a gas station), but it is really hard to mix that stuff in correctly (possibly one reason people can get growth) -- read the instuctions; just dumping some in the fuel port before filling is not adequate. If topping up from a jerry can, I would still recommend filling the jerry can from the fuel dock (way more expensive than the gas station, but.. biocide truly properly mixed in.) I do belt-and-suspenders, actually: fuel dock fuel AND biocide in the fuel port before filling : )
My tank is on the small side, 18 gallons.