I had some algae in the tank, just grows from the condensation. Sitting with little use seems to invite it. Looked at the auto supplys for a biocide to add to the fuel with no luck finally went to West Marine and they have plenty. When you kill the algae it seems to make its way to the fuel filters, so you have to change them a few times afterward.
But if the tank is real bad you may have to clean it out. I always like the easy way first and hope it works. Changing filters as opposed to dropping the tank is easy.