I have just cleaned (it was a try) as much as possible my diesel tank this summer by a dieselist from perkins La Rochelle. My SM isn't either a 2000. The access was only possible from the bottom of the tank. He took out the valve and put a new one in. When it was deinstalled I could try to cratch the black stuff out. You will never get all out! For this I have a big filter (300 micro meters) and a second one with 30 micros meters.
Before the dieselist let clean the dirty diesel by pumping out into a clean temporary tank with anti bug and push it back after have been holding through the night in the cleaning tank.
Hope this helps for the first time.
Good luck in Cleopatra Marina. I was there in 2000 during winter. Good price and good service.
SM 124, Félicie, Switzerland