Algae spores land in every pool, every day. Chlorine kills them before they take hold. The moment free chlorine drops to zero, and in a San Fernando Valley summer that can happen in two or three days, the algae gets a head start and the water starts to tint green.
The usual triggers are simple: a pump timer that got bumped, a week away without service, a heat wave that burned through chlorine faster than expected, or a filter so clogged that water barely moves. Once the water is green, adding a single bottle of shock rarely fixes it, because the algae is using up chlorine as fast as you add it.
A proper recovery starts with the filter, since it has to trap everything we're about to kill. Then the pool is shocked at a strength matched to the algae load, every surface is brushed so algae can't hide in the plaster, and dead algae is vacuumed out to waste rather than pushed back through the filter. Most pools take a few visits to go from green to clear.
The last step is the one that matters most: balancing the water so it holds chlorine again. If your pool has turned, call us before the algae sets into the plaster. The sooner it's treated, the faster and cheaper the recovery.