“The most likely reason for the appearance of the green water in the swimming pools at Rio is algae. The cause of algae in swimming pools is the same as the cause in garden ponds and tanks, the algae grows due to plentiful sunlight and nutrients in the water.

"However there is one crucial difference between algae management in fish ponds and swimming pools, specifically the employment of disinfection (usually by chlorine), in swimming pools which is supposed to kill off green water algae. Clearly such disinfection would be disastrous in a fish pond…

"Hence the management of algae in fish ponds and swimming pools is utterly different, though one parallel exists – the use of flocculents like Algorem or CrystalWater.

"I would imagine what has happened in Rio is a plant failure or human error with the disinfection system, not enough chlorine has gone into the pool. Also the chlorine dosing regime is pH and hardness dependent, therefore the pool manager should be monitoring these important parameters and dosing chlorine accordingly. Also the chlorine is unstable in the water hence regular redosing is vital.

"Once algae establishes in a pool it is hard to get rid of, (just like in a pond), the treatment involves elevating the chlorine level, (not good for the divers), using a flocculent to clump algae together then vacuuming and filtering out the clumps.

"When an algae bloom strikes in a pond, obviously disinfection is not an option, so the pond keeper can use herbicides, flocculents, and ultraviolet clarifiers to remove the algae, they then have to test the water quality and manage accordingly to lower the nutrient level to prevent algae occurring again. Shading can work aswell.

"So what parallels between green water in Rio and green water in a garden pond; if we assume the green colour is due to algae, then algae growth can be prevented in both cases by monitoring water quality and installing and managing properly a filtration and water treatment system.”