Thanks Wolfe 10 for the info. The CAT Tech told me it was Algae. It's dark, but looks like little particles in the clear bowl under my first fuel filter. It was recommended to me to use Biobor, as it is suppose to be one of the best additives on the market to eliminate water & Contaminates in my fuel tank. The CAT Tech used Sea Foam, to clean out the tank, but I was unable to find it readily. I was able to find Pri-D, but maybe it's not doing the job. What do you think? Thanks...