Microbial Contamination in Fuel Tanks: More Than Just "Algae"

Microbial contamination in fuel tanks gets called "algae" casually, but that's not usually accurate. What's actually growing at the fuel-water interface is typically bacteria or fungus, not algae. The distinction matters less for naming purposes and more for understanding what conditions cause it and how it behaves.

What it needs to grow

Microbial contamination establishes wherever water sits against fuel for an extended period, particularly in warmer conditions. Diesel is more commonly affected than gasoline, since diesel's chemistry supports microbial growth more readily. Once established, microbial colonies can persist and spread through a fuel system if the underlying water problem isn't addressed.

What it looks like

Microbial contamination typically produces slime or a dark, stringy debris that clogs filters faster than normal. It can also generate acidic byproducts as a metabolic waste product, which contributes to corrosion in tanks and fuel system components over time. Recurring filter clogs, especially paired with visible dark residue in a fuel sample, are a common sign.

Addressing it

Fuel polishing removes the water and much of the physical debris that supports microbial growth, but active microbial contamination often needs biocide or additive treatment alongside filtration to fully address the problem. A fuel sample is the most reliable way to confirm what's actually present before deciding on treatment.