Put a fairly strong bleach solution in the tank, much more than a cup, maybe a gallon in a 25 gallon tank. Let sit overnite. Drain tank through taps. Repeat if necessary. Check the hoses to the taps - they probably need replacing anyway - after all they're probably original. Then just fill and drain until the bleach is not apparent. It worked in my case and the tank hadn't been used for over a year. I replaced the lines as they were black with age and probably as old as the boat - 32 years.
ps - then you only have to get rid of the smell of the bleach.
Last edited by mitiempo; 11-14-2009 at 02:59 AM.