Diesel fuel whereabouts

[TheThunderbird]

In cleaning tanks, after sumping up dirtier and dirtier fuel (separated from water, filtered 3x or more, and back to transparency) I ended up with black mould (no surprise!) and one gallon of fuel which, after some 10X filtering, is still 85% close to full black opaqueness, though transparent when observed in minimal quantity, and "clean".

What does make it that dark !?

I'll throw it away, but need to understand...

Thanks:-)

Yap, I also cleaned hoses and curves.

[dlymn]

moisture in fuel alows algae to grow. Some components might be smaller than your filters and so are not filtered out. Keep tanks full add biocide install fuel polisher and multiple filters. rigorously maintain.