As an engineer who primarily now does filtration, polishing the tank without actually getting inside and scrubbing the walls to loosen up all the agglomerated crud will probably be a waste of time. Polishing will indeed clean up the oil but wont touch the debris thats adhering to the walls. Such attached debris is subject to come loose the next time you encounter a heavy sea state.
First, remove most of the fuel (use in you oil burner at home) then If its a removable small tank, simply take it out and do the cleaning work outside the boat; if large or has a large inspection port, simply get in there with a long handled stiff brush and scrub until all the junks is in the bottom, then ''sop it up'' with towels - then burn.
How to keep a tank from particulating: only keep on board the amount of fuel that you ''need'' plus an amount for ''emergency'' reserve. Fuel has a relatively short ''shelf-life'' and will begin to ''grow''/ agglomerate particulate because its ''old''. To keep particles from old fuel from forming you need: a clean tank, buy only from a high-turnover source (one that pumps a LOT of fuel), ..... and then consider to add an onboard recirculation/polishing systm to keep the tank clean.
Hope this helps