Gunk in the fuel tanks is not a problem unless it breaks loose and gets caught in the off-on switch or your filters. Put a in-line strainer ( not a filter) in the line to catch the big lumpy bits without blocking the line.
Biocides generally kill the bug but may not remove any gunk. Use an enzyme to break up the gunk slowly over a period of time. I have seen Startron remove a big lump of gunk from an in-line strainer after 9 hours of motoring without blocking filters. In fact the filters also appeared cleaner.
If you want to get to the bottom of the tank, remove the fuel pickup or the fuel gauge and insert the tube from a oil pump and suck out the bottom of the tank. Won't remove the gunk on the walls but will remove any water or solid stuff from the bottom of the tank.
Fuel polishing is unlikely to do much as the gunk may not be in the fuel. Also, I'm not convinced a fuel polishing system is of any use as your fuel pump, pumps up to 40 times more fuel than the engine uses, then returns the unused fuel to the tank, effectively polishing the fuel through your filters. Fuel polishing systems only appear to be useful in multitank systems on larger boats when pumping between tanks.
As mentioned in previous posts, many boats in Malta do not use any additives, never clean their tanks but remove their in-line strainers when they get filled with gunk. ( Once every year or two) Their tanks are full of gunk.
Craig
"Shirley Valentine"
Gold Coast
Australia