Please excuse me for resurrecting an old thread but I think I might be able to save someone some trouble by covering some of the issues.
The EVIL BLACK SLIME is algae and once you have it it can be extremely difficult to eradicate. You can clean a fuel can with some gas, letting it dry, and soak with PURE bleach which will kill the algae.
BLEACH DOES NOT GO AWAY THROUGH EVAPORATION!!!
The water evaporates out of it leaving the full strength chemical behind. After the bleach treatment you need to rinse well with water. Fill the jug 10% - 20% or so, shake vigorously and dump. Repeat this at least 3 times.
A good algicide (Biobor is one) should be added to the fuel as directed. Typically the first application is a double dose to "shock" the algae.
A fuel can with algae can contaminate any tank to which it passes fuel. Some folks use algicide as a preventative rather than a cure because the consequences are so dire. If the algae finds good growing conditions it will plug filters big time and can clog injectors and in general make diesel life miserable. You can get algae in your fuel when you buy it. You will not see it if it isn't "blooming."
Folks used to think algae only caused problems in long term storage so they only used algicide for their storage tanks. You can get algae in diesel at any time but whether or not it grows a lot mainly depends on temperature and water. The algae needs food and water to multiply. Diesel is the food and condensation in the fuel tank supplies the water. At moderate temps you then have ideal algae farming conditions.
Virtually all tanks vented to the atmosphere "breathe" in and out with changes in air temp and damp air can condense water in the tank which can let a microscopic "dose" of algae suddenly multiply like crazy.
I have seen many medium to extremely severe cases of algae contamination. Many were boat related but I have also personally had the problem in automotive diesel fuel. tank. A bad case will clog filters as fast as you can change them and can require compleat disassembly of all the fuel system for cleaning of all pumps and lines.
The thing to be aware of is that you can get diesel with algae in it and not know unless it grows a lot. The cost and hassle of prevention is much much less than the consequences of a bad infestation.
Pat