Green water is often referred to as "pea soup". Free floating single celled algae grows at such a rapid rate that it turns the water green.
If one were to take a glass of water, then added some fish food or plant fertilizer and placed it into a sunny spot the water would turn green within days... green water!
This "recipe" holds, in part, true for aquariums as well.
Green water is nothing more then single celled planktonic organisms that thrive on high dissolved organic matter and nutrients, often indicated by elevated nitrate levels. The most common type of blooming "algae", Euglena, consists of hundreds of species. Planktonic algae are unicellular organisms not clearly characterized as plant or animal. As all protists they have both, plant and animal like characteristics.
Equipped with flagella the protist is mobile, allowing the organism to move towards the light. Phothosyntesis is one of the energy sources this organism utilizes.
An organism that creates its own food is referred to as autotroph, while a heterothropic organism requires an organic substance for its development. The green water causing algae can switch from autotroph to heterothroph depending on the main nutrient available.
If light is not available for phothosyntesis the algae feed on dissolved organic matter. In cases of a longer absence of light, the chlorophyll will turn clear and regain its color once sufficient light is again available. In older literature the lights-out-method has been recommended to eliminate an algae bloom. The result in most cases was a return of the bloom within days if no other corrective measures have been taken.
Green water causing algae are always present, but will not bloom if the aquarium is well balanced, meaning that all things exist in an equilibrium. External or internal impacts will result in an imbalance.
In reference to the green planktonic algae the balance is maintened by either sufficient numbers of zooplankton, or low nutrient levels. Huge or frequent water changes will further reduce the zooplankton content and therefore increase the density of the green water by simply removing the leading predator. High nutrient levels are man made, by habit and circumstance as further described below.
Plankton is the base of the food chain. Green water causing algae are referred to as phytoplankton (microscopic plants/animals) that photosynthesize. Zooplanktons are microscopic animals that feed off the phytoplankton, linking the primary producers to the higher trophic levels. Small fish, larvae, and corals in reef environments then consume zooplankton.
Planktonic algae can therefore be kept in a balanced state if zooplankton is present. If not controlled by zooplankton, nutrients need to be controlled to avoid an explosive growth causing green water.
Zooplankton is part of the aquatic microfauna in established tanks. Algaecides and medications can be detrimental for the balance of phytoplankton and zooplankton. Destroyed in large numbers, zooplankton will no longer feed off the planktonic green algae, but further contribute to a nutrient accumulation, progressively 
fueling the algae bloom.
Nutrient accumulation next to an incapacitated microfauna will eventually trigger and fuel the green-water algae bloom.
Accumulated nutrients are a direct result of either overstocking an aquarium, overfeeding the system, inadequate filtration, and/or insufficient maintenance.
Before the nitrifying bacteria can convert ammonia to nitrite / nitrate, heterothropic bacteria consume dead organics such as uneaten fish food, feces, plant leaves etc. The heterotrophic bacteria sometimes referred to as scavengers, reproduce rapidly every 15-20 minutes, while the nitrifying bacteria need 10-15 days to multiply.
In new setups, heterotrophic bacteria get to work faster then the nitrifyiers, which in consequence leads to an accumulation of nutrients. In some cases the scavengers cause the water to become cloudy, which in this case is a bacteria bloom.
Heterotrophic bacteria settle wherever dead organic material can be found. In some cases they appear as a cloud after the aquarium substrate has been vacuumed or otherwise disturbed.
The more waste is in the aquarium, the larger the heterotrophic colonies will grow. During the mineralization process, ammonia, CO2, and dissolved organic matter are produced. CO2 of course is needed for photosynthesis; ammonia and dissolved organic matter are the other ingredients for the planktonic algae to thrive on. An increase of CO2 is directly linked to a declining pH.
Bacteria, either nitrifying or heterotrophic, require huge amounts of oxygen. As the water turns eutrophic (nutrient rich), an increase in bacterial oxygen demand can lead to insufficient oxygen levels for the fish. The lack of oxygen is the main risk during an algae bloom. This is enhanced by photosynthesizing planktonic algae, which produce oxygen during the day and deplete oxygen over night.
In almost every case green water can only be eliminated by lowering the eutrophication (the enrichment of an aquatic system by addition of nutrients) of the water column.
Algone, contains enzymes which break down proteins and carbohydrates, containing them in insoluble complex structures before mineralization through heterotrophic bacteria. Biochemical compounds oxidize organic/inorganic pollutants directly.
Removing and containing organic waste will consequently lower the dissolved organic content, and based on a regular lighting period (6-8 Hours) resolve the unsightly green water problem.
