Algae spores can first enter your pool through wind, rain, or even in the fabric of swimming costumes that have picked up microscopic spores from another area. Pool cleaning tools that are reused without being adequately washed after the last anti-algae scrub can also re-infect your pool with algae.
In a matter of hours, microscopic algae spores can rapidly multiply if the conditions for their growth are optimal. These kinds of conditions include warm temperatures, bright sunlight, and poorly balanced pool water with high levels of nitrates, phosphates, and carbon dioxide.
A lack of adequate circulation and filtration systems in your pool can also increase your risk of developing an algae problem.
Algae also grow better in pools that have plenty of food sources. Examples of algae food sources include soil, dust, and debris from the poolside, like crumbs, leaves or other plant matter.
Once you have experienced a serious algae bloom in which your pool becomes unusable, the chance of the same problem reoccurring at a similar level of severity is increased. This is because every pool, no matter how clean and green-free it may appear to the naked eye, will always have microscopic traces of algae in its waters.
After serious algae blooms, these microscopic traces are harder to eradicate, making the same problem more likely to reoccur. This is why putting effort into regular pool maintenance and never letting a small algae bloom get out of control are vital in the long term, as fighting back a serious algae infestation can be a very costly and time-consuming effort.