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Eastern European swimming pools are often brown tinted water. i was told it was the color of the chemical to keep the pools clean, but who knows. These pools did not smell unsanitary and may have even been the result of original brown listerine. May be it was just iron from a natural spring. The pool was still transparent as one could see the bottom of the deep end, but was definitely tinted brown. What would that chemical be? Would it significantly warm the pool by capturing more of the Sun's light?

These were traditional pools, not the more recent and highly popular natural swimming pools.

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Related question on skeptics: skeptics.stackexchange.com/questions/16277/… –  rjt May 15 '13 at 23:42
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Yes, colouring the water could make the pool heat faster, though whether the colour you noticed has an significant effect is debatable.

Swimming pools exchange heat with their environment by conduction through their walls, evaporation at the surface and absorption of sunlight. I have little direct experience of swimming pool thermodynamics, but some Googling suggests that evaporation is the main way heat is lost, and (if it's sunny) absorption of sunlight is the main way heat is gained. Presumably heat conduction through the walls is slow enough to be small compared to the other two.

The sunlight hitting the pool is proportional to its area, so that isn't easily changed. However the proportion of the sunlight absorbed is equal to the incident sunlight minus the reflected sunlight, and you can change the reflectivity. For example if you paint the bottom and walls of the pool black they will heat faster than if you paint them white, and the walls will then heat the water faster.

Under normal circumstances water absorbs relatively little light, so sunlight heats the walls and the walls heat the water. However if you put black ink into the water the sunlight will heat the water directly. So the black ink would decrease the overall reflectivity of the pool just as painting the walls black would, and it would increase the rate of heating.

So, the brown colouring you noticed will (probably) decrease the overall reflectivity and thereby increase the rate of heating. However if, as you say, the bottom of the pool is still visible I doubt the effect is very great.

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