Standing water in a white bathtub can sometimes appear to have a greenish tint to it. Fluorescent lights will make your water appear green. To test this, fill a white bucket with water and take it outside. In the sunlight, the water will look clear and no longer appear green.
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Another cause of green water is extreme copper plumbing corrosion. If this is happening, the water will usually have a bluish-green tint and may leave a bluish-green stain on porcelain if the water drips from a faucet. This copper corrosion can be caused by your electrical system being grounded to your water pipes; especially if you have a mixture of pipe material (e.g. some copper and some galvanized steel). If the green color is only in the hot water, it may be due to the temperature on the water heater being set too high. If you have a hot water circulating system, the return line may be too small or the water may be pumped too fast for your pipe size, or it may be installed incorrectly.