We cleaned the tanks in our Maramu and they were much worse than your, in fact we even had live snails living in there!  There was also significant amount of sediment in the aft low end of the tank.  I just used the bleach version of Soft scrub and a medium deck type brush with an extension handle to clean our tank followed by lots of flushing and pumping.  I was concerned that the abrasive powder in the Soft Scrub might be abrasive so I used a portable diaphragm pump to remove the water and debris instead of the ships water system. Obviously rinse the tank from forward to aft to flush things downhill.    Maybe be a little careful with the putty knife in case you encounter some loose gel coat, some came loose on my boat just from the scrubbing.

Best,

James

SV Sueno

Maramu #220