On any of the Algae sites all of the "informed" ones state green water is caused by the reasons stated in my post,
here's a info from just a couple :
## Green water (algae bloom)
This is the most common problem if the cloudy situation extends beyond 10-14 days. Note that "green water" (GW) is not always green in appearance! Since GW is the most common problem and the most difficult to solve the answer needs to reflect several options. The situation that causes GW is usually a combination of high nitrates, phosphates, and mixed in some ammonia/ammonium. Substrate disturbance is usually the culprit. What happens is the algae (GW form) will flourish off of the ammonia/ammonium and phosphate, remembering that algae can consume phosphate easier than plants because of their thin cell walls, the algae uses up the ammonia/ammonium and phosphate, but it doesn't go away...because algae can quickly switch which nutrient it scavenges...it moves to nitrates. So you can see why water changes will not rid a tank of GW. Nutrients can be reduced very low in GW and fairly quickly by the GW algaes, but they can scavenge other nutrients...iron and trace elements. So, it's very common for the GW to solve the situation that causes it to begin with, but that won't eliminate the GW, for the reasons I've alluded to. Five methods exist to eliminate GW. Blackout, Diatom Filtering, UV Sterilization, Live Daphnia, and Chemical algaecides/flocculents. The first four cause no harm to fish, the fifth one does.
Description This is a unicellular algae. Water goes cloudy. Sometimes just a green tint, other times it can look like pea soup.
Cause Ammonia is often the main cause green water. There may have been an ammonia spike that isn't detected with test kits. Other possible causes are an imbalance of nutrients and/or low CO2 levels.
Removal Large water changes do not seem to always help. If there is an imbalance in nutrients then fixing it will sometimes make it go away by itself after a while. A three day blackout followed by a large water change will hit it hard and sometimes may clear it. A UV steriliser/clarifier or diatom filter will clear it up very quickly and is often the only way to clear it.
A new method is to use freshly cut 1-2 year old willow branches about 0.5-1cm in width. Place these in your tank vertically so they go from the substrate to a few centimetres above the water's surface. After a few days they will start to grow roots and the green water should start to clear. When cleared remove the branches from the water.
Don't confuse this with a bacterial bloom which gives the water a white haze.
The cause of green water is not due to exposure to daylight.
The use of UV will help reduce or eliminate it completely, again as I stated in my 1st reply
and yes I've suffered with green water in my aquariums quite a while back and it wasn't exposed to any direct daylight it was caused by excess nutrients mainly due to overfeeding, and yes I quickly resolved it with an external TMC UV steriliser.