• Here are the top 10 causes of green pool water (ranked by how often they happen in real-world pools) and how to fix them:

1. Algae Growth – The #1 cause. Algae thrive when chlorine is too low or circulation is poor.  FIX by shocking the pool or adding an algaecide.  Just be careful with copper based algaecides as they add copper to the water (use a CuLator Metal Eliminator to remove the left-over copper) 
2. Low Free Chlorine Levels – Without enough sanitizer, algae and bacteria quickly multiply, turning the pool green.  FIX by shocking the pool and keep the free chlorine level between 1-3.  You may need to keep the chlorine level elevated until the water clears.
3. Poor Filtration or Circulation – If the filter is clogged, undersized, or the pump isn’t running long enough, algae spores settle and bloom.  FIX by cleaning the filter(s) and check the pump.
4. High Phosphates/Nitrates – These act as “food” for algae, accelerating growth even if chlorine is present.  FIX by using a phosphate remover or drain some of the water from the swimming pool.
5. Imbalanced pH – High or low pH can reduce chlorine effectiveness, allowing algae to take hold.  FIX by raising or lowering your pH.  Salt systems often have higher pH.  Check the pH regularly.
6. High Cyanuric Acid (Stabilizer) – Too much CYA locks up chlorine, leaving the pool under-sanitized despite good chlorine readings.  FIX by training some of the water from the swimming pool.
7. Metals in the Water (Copper/Iron) – Oxidized copper can give a green tint, especially after shocking. Iron can also cause discoloration.  If the pH goes up too high, then metals may “plate out” onto the surface as a stain.  FIX by using a CuLator Metal Eliminator to remove the metals and prevent stains.  Also you can prevent staining by using a sequestering agent like Mega Maintain.
8. Warm Water & Sunlight – Heat and direct sun speed up algae reproduction and chlorine breakdown.  FIX by checking your chlorine levels regularly.
9. Organic Debris – Leaves, pollen, dirt, and other matter feed algae and deplete chlorine.  FIX by removing debris frequently and running the filtration system regularly.
10. Low Alkalinity –  If your alkalinity is low, you may get green water, which is also aggressive water.  FIX by raising the alkalinity to 80-120 for liners and 100-125 for plaster.

NOTE: Most of the time, green water is a chlorine and algae problem, but metals (especially copper from algaecides and damaged heaters and iron from salt systems, bricks, pavers, or stones around the pool, screens around the pool, run off from irrigation, and well water) can also be the culprit.  If your water is clear, but green, then you most likely have a metal issue. If the swimming pool water is cloudy, and green, that is an indicator that you need to add chlorine to the water to get rid of the cloudiness and then treat the metals.