Why did the Olympic pool go green and why was it so quick?

http://m.bbc.co.uk/sport/olympics/37029833
That article mentions Alkaline. How does that affect the colour? And would it really be safe?

Oddly enough, it's still not completely clear. The most definitive information from an official source comes from the international aquatics ruling body, which said:

FINA can confirm that the reason for the unusual water color observed during the Rio diving competitions is that the water tanks ran out of some of the chemicals used in the water treatment process. As a result, the pH level of the water was outside the usual range, causing the discoloration. The FINA Sport Medicine Committee conducted tests on the water quality and concluded that there was no risk to the health and safety of the athletes, and no reason for the competition to be affected.

The problem is that this answer by itself doesn't really explain the change in the color. Since there is little more official information, the rest of the answer has to be a bit more speculative. In this article a pool maintenance director suggests two explanations: the pH imbalance could have caused 1) an algal bloom or 2) it could have dissolved minerals in the water causing its coloration. The algal bloom theory was the one first offered by Olympics officials as well. The only problem is that it seems unlikely that it could have kicked in as fast as it did. The other possibility is also feasible. We know that mineral rich bodies often have a natural green/blue color due to dissolved inorganic species such as copper ions.

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