Ah yes Possel, The dreaded cladisporium I remember it well !
Many years ago we had a Laker BAC 1-11 ( That tells you how long ago
) that had operated with Zambia Airways.
That came back to UK and had to have wing planks replaced because of this.
Cladisporium is apparently very difficult to filter out, however the engine will happily eat it if it goes through. It is more active with higher temps hence the tropics problem. At altitude (-50oC) it goes dormant and then comes round when the temp goes up.
We were told to make sure tank drains were religiously checked at all ground times especially in tropics.
We did not use additives in the big jets but we used Biobor on the Lears. ( guess in those days those liitle spray cans which we used on the Lears would have been a bit of a pain on the big jets
The use of jet pumps on later aircraft is also to swirl the low part of the tanks and make sure that the fungi is eaten by the engine and not left to breed in the tank bottom.