The best way to do it i to remove all the fuel and, if you have access ports
, physically clean out the tank.
Don't believe the hype about fuel polishing as that, while it does clean the fuel of suspended stuff, any of the crud on the bottom and sides, stays there.
No additives are going to do a thing for you right now.
Nor will an Algae-X......Personally, I think they are a scam.
Magnetism does not kill bugs, otherwise we would use MRIs to cure illness.