I had my fuel pumped and tank cleaned by a professional outfit in Florida
but mainly all he did was agitate a bit and vacuum out fuel then agitate some more and vacuum. There was only about 8 gal of fuel and it looked awful so we discarded it. Later, in the Turks and Caicos
we had to remove the tank to do some work under it so we pumped the fuel through filters into a plastic 55 gal drum. Removed the tank and cleaned it well with a pressure washer, upside down, on its side, etc. and let it dry for two days in the sun. I have a very small access on the top of the tank, large enough for a pressure washing
wand but too small to get your arm in there. Then we replaced the fuel by pumping again through filters. I'm glad we got the opportunity to clean it well. Since then I've been even more careful to filter fuel and add a bioagent.
If I understand correctly, bacteria and algae can grow in the fuel, even in the dark, and when they die they sink to the bottom. I don't think that they are "attached" to the tank itself but they tend to clump. So when agitated in rough seas and bad weather
the clumps start floating around and cause all kinds of problems.
I'd look very carefully to see if it is possible to remove the tanks. Although mine was a pain to get out, I found that cleaning
it on the dock
was a lot easier than working in the cramped quarters below deck
and breathing the inevitable fumes.