Years ago, Dad got that algae/slime inside the JD 6620 combine's fuel tank. It is only piece of machinery that gets filled with the portable 110 gallon pickup tank.(the 110 gallon tank was the actual culprit) 

No other tractors had a problem = they only get filled from farm's 500 gallon storage tank.

We had to remove Combine's fuel tank , because it was so bad = looked like black slimy "lilly pads" floating in that combine fuel tank (looking thru fuel tank sending unit hole)

Used pressure washer (to wash inside tank), and compressed air (to blow out), and tied shop rags, onto #9 wire to dry out the inside, where I couldn't reach with my arms.

Note: No way, was any store bought liquid or biocide going to clean out this mess ( that stuff had 10 months to grow, while combine was in storage.)

Sucked out 110 gallon tank, as best we could, installed some biocide, and a little diesel and installed new fuel filter on that 110 gallon tank and circulated "treated diesel" thru the 110 gallon hand pump and let it sit, so the biocide could "do its job". Note: this 110 gallon tank is stored inside, and empty, each year; but I'm sure there is a little fuel in a corner, PLUS condensation, for 10 months, as it sits idle.

Ever since then, I treat every new  500 gallon load, of diesel fuel with just a slight amount of biocide(way under the recommended amount, listed on the  bottle) and we've never had another issue.

Each Fall, I also add about 10-15 gallons of diesel to 110 gallon pickup tank with an additional dose of biocide, added. I circulated the hand pump and let sit for 1 week , before we totally fill it, and begin using it.

Probably spend $50-$60 on biocide, a year = for prevention.