It's Not Really Algae
Pardon if this is a trivial point, but in the interest of all of us increasing our knowledge...
Whatever is growing in your diesel tank, it's not algae. Algae requires light to grow, and there's precious little of that in your tank. The gunk that grows there is usually a mixture of fungus and bacteria. While they don't require light, they do require some moisture, and grow most readily at the fuel/water interface.
Thus, I don't trust any product that claims to kill "algae". The better ones are called "biocides", and kill "microorganisms" in the fuel. Probably the worst offender is the sham product called "Algae-X", which is simply a magnet in a casing that it tied into your fuel line.
In an interesting twist, there is some research have luck using certain types of algae to produce diesel fuel. So I guess while you "can't have algae in your diesel", you "can have diesel in your algae"!