Algae in a swimming pool presents a problem for a pool owner, mainly because it makes such a mess of a pool's water. Algae are microscopic plant forms that require water and sunlight, at minimum, to grow. A swimming pool is a perfect growth environment for algae because it gives them water and sunlight in abundance. Algae are like any other land-based plant, however, in that they need light to survive. Therefore, if you cut pool algae off from light, they'll soon die.
For swimming pool owners, the need to prevent algae from taking up residence in their pools is an important maintenance task. Swimming pool algae prevention depends on having consistently clean water, for one. Preventing swimming pool algae can also be supplemented, though, by keeping a pool covered with a black or very dark pool cover or tarp. Algae need light to create their energy through photosynthesis, and if you stop them from getting light, they'll die within a day or two.
A pure black pool cover or tarp will quickly block sunlight and deprive algae of the light they need to survive, thereby killing them. However, pure black pool covers or tarps have a big drawback. A swimming pool tarp, for example, that's black on both sides is like a giant solar panel. Because a black tarp on a swimming pool's water surface traps solar heat energy, it will eventually turn cool, refreshing pool water into a tepid, warm bath.
Prevent swimming pool algae from appearing or kill them off by using a cover that's black on one side and sunlight-reflective on the other. As of July 2011, there are swimming pool cover manufacturers that offer specially designed algae-inhibiting pool covers. Algae-inhibiting pool covers feature reflective exterior surfaces that turn away the sun's rays so the algae are deprived of sunlight. An algae-inhibiting swimming pool cover's reflective color on its top, or nonwater side, prevents heat absorption and keeps water comfortably cool for swimming.
Covering a swimming pool with a black tarp to kill off an algae contamination can take a couple of days to accomplish. Even after you kill algae in a swimming pool, you'll still need to clean up the pool and superchlorinate it to ensure complete algae eradication. Often, dead algae are too small for pool filtration. After killing off pool algae, apply a pool water clarifier to clump them up so that they're large enough to be filtered out or vacuumed up.