Roger

Glad you noted the crud in the bottom of the Shelco housing.... this
debris is/are the nucleation sites upon which such particle grow/
agglomerate.
Full or topped-off tanks - never unless Im travelling. Water in oil
is in two phases: 1. sensible (free) water that drops out by gravity,
2. water that is emusified (bound) in the oil.
Fuel oil that is long term exposed to atmosphere saturated with water
(from vapor) will eventually settle out in the bottom of the tank. I
never ever 'top off' a tank unless I plan to use most of it in the
short term. Why store the oil in a tank with an atmospheric vent only
to have it uptake water (vapor), etc.? Simple solution is only upload
what you need plus some reserve; if storing long term, drain the oil
and burn it in your home heater as EMPTY tanks dont 'condense'
water.

The probable chief organism that thrives in fuel oil tanks is
Cladosporium Resinae - ***a fungus*** that is spread by spores ...
most probably aspirated through the tank VENT pipe. Currently Im
using a bio-blocking 0,2µM abs PTFE membrane filter capsule on my vent
and will compare the tank innards on my next scheduled clean-out (in 2
years). Million gallon field fuel tanks rarely become contaminated;
its the teeny boat tanks that usually have the particulate/
contamination problems.

Im currently travelling and using a lot of fuel so Ive temporarily
removed the desiccant vapor trap on the vent line.

regards to all.