While I am not a pool expert I am asked more and more often to work on pools in gardens I maintain.
And most calls come about because the once inviting swimming pool water has turned a bilious shade of green.
The usual knee-jerk reaction to this is to dose the pool with a couple of shock treatments and crossed fingers.
In my experience the real problem is not the level of chlorine in the water.
The secret to clear pools begins with a maintained PH level. It’s my base point and nothing else gets done until that is stabilised and I like 7 to 7.2 as a base line. Algae enjoys higher ph levels and ph levels tend to drift upwards if not managed. Adding huge doses of chlorine to the water simply accelerates that upwards trend and so actually helps the algae flourish.
So bring the pool water into the right PH range and keep it there before throwing money into your pool water.