Green Water

[Liv15]

What does it mean if a tank has been set up and running fine for a few months and suddenly the water turns green. Water changes and 'green away' treatments (that take the phosphates out of the water which algae live off of) do not work. Tank is not in direct sunlight, ammonia is up slightly and water is hard.

 

[Byron]

Green water is caused by unicellular algae which increase due to high organics (phosphates are often blamed, but nitrates ammonia/ammonium, nutrients in general can be used as food too) in the presence of light that is likely high in intensity and/or duration (meaning, beyond what is necessary for the plants if planted, or just too much period).  Overfeeding and excessive disturbance of a "dirty" substrate can also trigger an algae bloom.
 
Water changes will not usually help, because they do not deal with one of the main causes, light obviously.  They may help by reducing organics/nitrates provided these are not in the source water to begin with.  Chemical treatments are detrimental to fish, so these are best avoided.  UV will kill unicellular algae, but before acquiring a UV unit I would do some tests to ascertain what is out.  Nitrates, pH, overfeeding, fish load, etc.
 
Byron.