50 Causes of Swimming Pool Algae

dog-days-of-summer-2Currently, the Dog Days of Summer Sale is running here at In The Swim, which reminds me of an old phrase – the “algae days of summer“.

It’s the time of year algae has been waiting for. As water temperatures soar into the 80’s, your sanitizer struggles to keep up – ideal conditions for algae.

But there are other causes of swimming pool algae. So many in fact, that I’ve curated a list of what causes algae to grow in pools. In most cases, it’s not just one factor, but a combination of many…

What Causes Pool Algae?

Water Chemistry Problems

  1. Low free chlorine levels, below 1.0 ppm
  2. High combined chlorine levels, above 0.3 ppm
  3. High pH levels, rising above 7.6
  4. Low calcium levels, below 150 ppm
  5. Low alkalinity levels, below 70 ppm
  6. Low cyanuric acid levels, below 20 ppm
  7. High cyanuric acid levels, above 100 ppm
  8. Very high total dissolved solids, over 10000 ppm
  9. Never brushing your pool; ≈ weekly
  10. Never diluting the water; ≈ annually
  11. Not shocking the pool enough; ≈ monthly
  12. Not testing and balancing the water enough; ≈ weekly

    Pool Water Contamination

  13. Dead remains of previous algae blooms
  14. High levels of dissolved carbon dioxide
  15. High levels of nitrates or nitrites
  16. High levels of phosphates
  17. High levels of bacteria
  18. High levels of ammonia
  19. Dirty pool, lots of leaves, debris
  20. Heavy swimmer use, bodily wastes
  21. Mulch leaking in or blowing into pool
  22. Fertilizers carried in from rain or wind
  23. Run-off from nearby lawn or planters
  24. Sap and pollen from overhanging trees
  25. Urban pollution from cars and planes

    Equipment Problems

  26. Pump is overheating and cycling on/off
  27. Pump is too large for the pool filter
  28. Pump impeller is clogged or broken
  29. Leaking or low water levels, drawing air
  30. Stuck skimmer weirs, drawing air
  31. Air leak before pump, drawing air
  32. Poor circulation in a large pool
  33. Filter is too small for pool size or pump
  34. Filter cartridge not seated properly, or missing parts
  35. Filter cartridge or sand needs replacement
  36. Filter not running long enough daily
  37. Multiport valve is recirculating or bypassing filter
  38. Suction or return valves are closed
  39. Salt chlorinator cell needs cleaning
  40. Salt chlorinator cell needs replacement
  41. Salt level in pool is too low
  42. Tablet chlorinator is clogged
  43. Tablet chlorinator is air locked

    Other Problems…

  44. Rough concrete pool surfaces
  45. Gravel, plants or fish (!) in the pool
  46. High water temperature, over 80° F
  47. All day sun exposure, over 8 hrs daily
  48. History of persistent algae every spring and summer
  49. Dogs, ducks or other animals using the pool regularly
  50. Non-pool items in pool; furniture, bicycles, skateboards, shoes, fruit…

pug-in-poolIt’s often not just one thing that causes pool algae to bloom, but a combination of factors that come into play.

To sum it all up, keep your water balanced and sanitized, make sure your filtration and circulation is optimal, and keep contaminants away from your pool. Visit our Algaecide section for targeted treatments.

Davy Merino
InTheSwim Blog Editor



50 Causes of Swimming Pool Algae — 9 Comments

  1. my total dissolved solids (TDS) are 3000 and that is why the pool chemicals are not working properly. they are telling me at the pool store I have to drain down the pool and add new water. Is this the only way to reduce the TDS. Is there any other way to reduce this number without draining. (Ive had the same filter for about 16 months)

    • hi luisa, TDS can cause problems for chlorine and pH, yes. Pools with fresh water are always more manageable. When possible (not always in arid regions), I like to replace pool water every 5-7 years, if it is not flushed enough by rain, backwashing and winterization. So, pools with cartridge filters, in arid areas, that don’t winterize, or heavily used indoor pools and spas – tend to have TDS problems at some point, especially those in hard-water regions of the US. There is no chemical to lower TDS levels in pools, The only other way to solve the problem would be to have the water cycled through a truck-mounted reverse osmosis filter, which will remove most of the TDS. In some parts of the southwest, there are companies that specialize in “pool water recycling”, and provide this service. 3000 is fairly high, but not super high, but high enough to make it harder for chlorine to do it’s job. I concur with the store however, and would replace 1/4 to 1/3 of the water, or more if possible.

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  3. Can spores released from algae growing in our pool cause some type of mold or fungus to grow on other outdoor surfaces? (Such as our wood pergala, or the wooden privacy fence)

    • Hi Tabitha, I’ve never heard of algae doing that, but algae spores are everywhere, and anything wood is a target for airborne parasites like algae, moss, mold, etc. Probably not related to pool algae.

  4. Our pool has been greenish for weeks, due to the green nuts from our 2 palm trees. We have shocked, back washed, rinsed, added chlorine, blue gel for cloudiness, brushed entire pool, vacuumed, done just filtering, waiting, moaned, begged – all MANY times. Tests are all normal and there is NO algae, but some green nuts. PLEASE HELP URGENTLY, WE NEED TO RENT OUT OUR COTTAGES AND THE POOL LOOKS LIKE HELL!!

    • Hi, yes, tannins, pollen and other colored substances from leaves, nuts and such can turn the water colors. A DE filter will usually be able to remove the very small particles. If you have a sand filter, you could add some DE powder to the filter, thru the skimmer, just a cup, as a ‘Filter Aid’. Aluminum Sulfate (Alum) works even better. Both backwash out of the filter, and can be re-charged as needed. If you have a filter cartridge, and you haven’t replaced them in a few years, new filter carts may be the trick. The Slime Bag, is a filter bag that connects to the pool return. Could also be helpful to trap colorants or tannins. Enzymes would also work, I would think, like Pool Magic or Pool Perfect or Pool First Aid, from Natural Chemistry.

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