FWIW
My boat stores up to 600 gal of diesel. Last year we didn't go out from October 1 to June 1. At the end of the season I had over 1/2 a tank left. That fuel sat there undisturbed all winter. I usually fill up in the fall, but didn't get to it last fall, so it sat there with lots of opportunity for condensation. Condensation provides the media for algae growth. I haven't had any trouble with algae at any time in the past 18 years that I have owned this boat, but I always feel better when my tanks are left full.
I have never added Stabil. In the early years I added some Biobor, but when I failed to do so, I didn't have any issues, so discontinued the practice.