The instructions on the chlorine jug assume your pool is in chemical balance. If it isn't, and most aren't quite, chlorine gets partly consumed by side reactions and doesn't kill algae effectively.
The four most common reasons algae returns despite chlorine:
Above about 80 ppm, chlorine becomes sluggish. This is the single most common cause, usually because the pool has been chlorinated with tablets, which add cyanuric acid as they dissolve.
Phosphates are algae food, and they enter the pool from rain, fertilizer, tap water, and bathers. Without removing them, you're fighting an enemy that gets fed every day.
Algae loves dead spots. Steps, behind ladders, and corners are usually the first sites of a bloom.
Brushing dislodges the early biofilm before it becomes visible. It's the single most effective thing a homeowner can do.
Hiring someone to care for your pool is a personal decision. Look around. Ask questions. When the time feels right, we'd be glad to hear from you.
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