If the tank is vented to atmosphere ... as you draw down fuel to run the engine, fungal etc. spores can enter through the vent into the tank. The 'most common' fouling in fuel tanks is probably fungi, which enter the tank as spores, and which 'eat' the oil as their nutrient source. Their products of 'metabolism' plus 'dead' fungi cells is the 'black' (resins) you see in a fouled tank.

A recirc. system constantly removes the spores (~3µM); no spores - zero to very little fouling from massive fungal, etc. slimes blocking the filter(s).