**Reddit**

Im in Mexico, and I just wanted to share with you, and leave this for the record and for if someone else gets trapped in the same situation. I manage a small, 21000 litre, 5500 gallon commercial pool. Twice Ive had unexplainable and sudden change of my water to green. Twice I had to change it. I kept blaming the municipal water supply, but after going around about the problem and this is a summary:

Had an extended chloramine problem, tried breakpoint to no avail. Then tried hydrogen peroxide to oxidize the chloramines, but with no UV since those installations are very expensive in Mexico. Worth mentioning that every pool store in my city only supplies stabilized chlorine. They do not sell liquid chlorine under any circumstances, and they dont even know what MPS or a metal sequestrant are. Thus, I buy my 13% sodium hypochlorite at an industrial site.

Peroxide did nothing, and I started killing it with with chlorine. 15 liters (4 gallons) in total over the course of two days. Then, in one hour and a half, it became green. And then started to get darker and darker. 

I spent solid 7 hours doing research, analyzing options, finding out that not a single pool store in my city had sequestrants. All the time blaming the water authority (which is to blame because we have the worst water quality in the broader region). 

And then, I got it. I remembered that my chlorine provider sent me one month ago the analysis of their batch of chlorine. 

IRON: 1.3PPM. I HAD JUST ADDED AT LEAST 0.5-0.7PPM OF IRON TO THE POOL IN LESS THAN 24 HOURS. Not counting the dozens of gallons I had poured over the past months.

Just as a cautionary tale to always ask for a batch analysis of your chemicals.

Green pool, unsuspected iron source.