If you have (white) road diesel in the tank, it's got 5% ethanol in it. Ethanol is hydroscopic (it absorbs water) and doesn't age well. It also attacks a lot of traditional sealing materials. If you have "proper" diesel, it will be low sulphur mineral oil with a few additives. "Propper" Diesel is fine and doesn't "age". Any water that gets into the tank will sit at the bottom, so siphoning out a pint or so should prevent proliferation of the dreaded "bug". A liberal dose of biocide will help kill off any residual bacteria. If you find black sludge in the bottom of the tank, too late, you have to clean the tank properly as this stuff causes all sorts of problems when it gets into the fuel lines. (I didn't say "if"). I have a tiny "sump in my fuel tank and there is usually a red powdery deposit here that appears in a clear siphon. I had mine tested in a lab when I first noticed it and it was residue from the red dye used to colour the diesel, together with some "fine inert particulates". (AKA dirt in the fuel)
Last time I had the dreaded "bug" I removed the tank and had it steam cleaned. It then leaked and I had to have to out, steam cleaned and welded up (it's SS). I should have had it tested when it was first cleaned. Lessons learned. I filled the tank two years ago at the end of winter. This year the boat has hardly moved from it's mooring, yet the engine started immediately when I was hauling out.
Condensation is not mythical. Metal tanks get cold in winter and any moisture in the air space will have vapour in it which will condense out as the temperature drops. This is why boat owners are often encouraged to top up at the end of the season. You never get much but it will permeate to the bottom of the tank and pool at the lowest point. Make sure your filler cap seals well, moisture more often gets in when it rains and this will be a bigger concern. I try to ensure my boat is propped up a little down at the bows and slightly listing to encourage rain water away from the filler as well. I also smear a little sealant (blue hylomar or Fernox LSX) around the edge of the filler cap as it's almost flush with my deck. The sealant is easily removed in the spring.
You won't get an engineer to weld a fuel tank that hasn't been cleaned and some will insist it be purged. Good HSAW practise.
1. Cowes Harbour commissioners fuel barge, just past the chain ferry does Marine Diesel with no Methanol.
2. The bug lives and grows in the interface between diesel and water. For you to have a bug problem you have to have water in the tank. Methanol in Diesel isn't a problem in motor vehicles because of the high turn over of fuel (My Rav4 uses a tankful every two or three weeks) My yacht seldom burns more than 25 litre as season, 35 in a good year (last year probably only about 1-2 litres). The tank holds 44. Were I using "White" Diesel I'd pump out the fuel tank and use it in the car, filling up at the start of the season and put a box of Dessicant in the tank while the boat's ashore. I've had three bouts of the bug and am not keen to have a fourth.
I suspect if you're using garage white, the methanol in the fuel will absorb any condensation but it does "go off". Adding fresh fuel means that you'll enrich the old fuel when you do it, but wouldn't like to rely on it. I suspect my old 1GM10 isn't critical when it comes to fuel quality anyway. I've a mate who uses road diesel in his boat but he buys the premium stuff from Shell or BP believing the extra additives help. I no longer have access to a fuel testing lab and have never explored the phenomenon.
