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The water has gone green because the algae have partied. This is because there is nutrients in the water, and lots of them. Eventually, the water will clear, because the nutrients will run out and the algae will die, but that might take while, especially if the water is being topped up by the rainwater guttering..........Charcoal has properties to absorb all sorts of stuff, and is usually used as a filter, but bunging lumps in a butt is unlikely to do much good, unless you chuck in about 20% of the volume of the butt.With ponds, one of the best ways to clear a green pond water is to throw a bale of barley straw in, this soaks up the nutrients .The best strategy is to make sure the butt is totally covered, and no light can get in. The other thing algae needs to multiply is energy, which it gets from sunlight. No sunlight, no baby algae.