About algae.
The algae are many, by the way, but a single one is an "alga" (with a hard g, like "Olga"). Saying "algaes" is like saying "feetses."
"Algae" is a convenient "Other" category that resulted when early biologists threw together as "plants" all the photosynthesizing organisms that made their own food from light, and then removed all the mosses and liverworts and vascular plants, calling the remainder "algae." In the residue were lumped together various photosynthesizers that had no common genealogy, no shared evolutionary history. But "algae" is still a useful working category, like "invertebrates," that other familiar miscellaneous "everything else" pigeonhole.
Biologists identify and set apart the broadest algal groups by the characteristic sets of photosynthetic pigment types each group has in common. Algae use photosynthesizing pigments that are much more various than the narrow selection the "higher" plants inherited from their green algal ancestors. The various chlorophylls are the energy-trapping pigments we're most familiar with, but there are other, auxiliary photosynthetic pigments, like carotenoids. The universally-used photosynthetic chemistry is powered by chlorophyll a. The green algae (Chlorophytes) use chlorophyll b in addition to a. The completely unrelated red algae (Rhodophytes) use no chlorophyll b but manufacture instead some chlorophyll c in addition to their chlorophyll a. So those Rhodophytes are as distinct in their metabolism--— and most probably as unrelated in their genetic history--— from green algae or from Chrysophytes (diatoms) as a Sequoia is from the fungus that grows on its trunk. There's no overlap in the chlorophyll chemistries that identify each group... and probably no shared family history, either. So the algae vary in their most basic biochemistry and the compounds they use for building cell walls and storing energy. Conveniently, the various morphological types of algae divide up pretty much the same way as their differing photosynthetic apparatus suggests.
Why should such biology trivia be of interest to you? Well, for one thing these various chlorophylls each have a slightly different absorption spectrum. So it strikes me that, if one of your tanks has chronic trouble with black brush algae or with cyanobacteria, a fresh fluorescent bulb with a slightly different spectrum might be just enough to tip the scales in favor of the higher plants. Or, fishkeepers have found, sometimes all it takes is switching the lighting in its reflectors between two tanks, without changing a tube, to set back the algae in each tank. If you want to outwit your enemy, it pays to know your enemy.
And now you know algae's dirty secret: algae are not a related group of organisms after all, just a convenient designation for photosynthesizing eucaryotes with cellulose cell walls, whether they are single-celled or form colonial filaments, sheets or spheres, or even the huge multicellular structures (called "thalli") of kelp.
Courtesy of the Skeptical Aquarist.