I’m NOT, or recommending anyone else, personally doing any hole cutting on a another guy’s boat, ever. Just making a suggestion for an owner to have it done if they want one way to get all the gunk, glop, sludge, algae crud out of their tank. It is why in the ‘olden days’ diesel tanks (particularly in marine environments and on boats) had access ports and panels. So they could get in there and actually get things cleaned out.
Opening up ANY FUEL TANK can be a VERY DANGEROUS AFFAIR if done improperly. I personally fill fully (over topping) the tank with water making sure there is no air voids and then only use non sparking methods to cut any opening. When responsible to having this done I’ve have at least one person on standby with an appropriate dry chemical fuel fire extinguisher and a third whose only job it is to call for help if it goes sideways. This is a process BEST LEFT TO PROFESSIONALS who are experienced and have the proper tools.
This is why I have a substantial dislike of ‘tomb’ tanks and designers who don’t provide a reasonable means to remove & service them. It’s a short sighted approach that leaves the future owner with a problem that WILL OCCUR as your friend discovered.
Getting a tank cleaned out throughly is the intent. How one goes about it is varied.
Most opt for a cleaning through the fill pipe as it is the only practical way with a floppy high pressure sprayer and a suction line to pull out the crud as it breaks free. All that is done in the blind as you can’t see where it got clean and where it didn’t. Even though a flexible borescope can help it still can’t see everywhere or direct where to clean.
Biocides May kill the algae but it doesn’t get the slime and crud off the tank walls and fuel lines.
Once you get the tank and fuel lines cleaned to the ‘owners’ satisfaction then trying to keep it clean in the future is an ongoing task.
Summary line from the filter article:
So, is it worth the hassle of filtering fuel at deck level? The tests strongly indicate that as a routine safeguard, the answer is a resounding “Yes,” and the filter of choice is the West Marine Model WM-F8C.
Hope this was of some help.
I apologize
if I misstated intent on opening a fuel tank. DON’T DO THAT if there aren’t already access ports or panels. Lots of large area filtration and sight bowls and more than regular changes help.Best Regards
Over Easy
Read accordingly. 
