Swimming Pool Blog

Pool Algae that Keeps Coming Back!

pool algae keeps coming back
by Mark Garcia June 29, 2016

pool algae keeps coming back
I like to call it 'recurrent algae outbreaks', a fancy way of saying pool algae that keeps coming back.

When you can't get algae to take an extended summer vacation, and green or yellow algae always comes back to your pool, 2, 3 or more times per season, it can seem like a never ending cycle. And it is, until you break the cycle!

To get rid of algae permanently you may have to take some drastic steps, but there are several things going on. Let's take them in order of probability then, shall we? 


Consider the lowly pool filter, daily dedicated to pulling out and expelling algae spores. If you have relied on your pool filter to extract algae from your pool water many times in the past, you likely have the cellular remains of previous algae blooms, still harboring in your filter.

Changing the filter media (sand, de grids or cartridges) goes a long way to stopping algae from returning. Especially for sand filters, backwashing won't remove it, and it's hard to get every trace of microscopic algae cells out of a cartridge pool filter. So, unless you just changed the filter media last year, replace filter sand after several algae treatments. Cartridges and grids (removed from assembly) can be soaked in a 10% bleach to water solution for several hours, and hosed deeply.

Another filter problem that causes algae to come back is not running the filter long enough. Or running the filter only at night. During the hotter months of summer, most pools need at least 12 hours of daily filtering, while the sun is shining, to keep chlorine circulating, and to remove small particles that cloud the pool, giving algae some UV protection. Algae will return again to the pool if you are only filtering 8-10 hours per day. Depending on the heat and how efficient your filter is, you may need 15-18 hours of daily filtration at certain points of the summer.


One of algae's favorite foods, phosphates and nitrates enter the pool naturally, all the time, but an accidental dose of fertilizer or small wash-in of soil and mulch, and your pool phosphate levels can jump sky high, a veritable banquet for algae to come back and feast. Since algae is a plant, any small amount of fertilizer that blows in or runs in from nearby lawns and planters will almost certainly cause repeated algae problems.

You can test your pool water for phosphates with a phosphate test strip, about $10, or you can spend about $30 and just go ahead and treat the pool with a phosphate remover chemical. Follow the label instructions carefully to dose correctly, and your algae problems could be over. Many pools that have algae that won't go away have high phosphate levels (over 300 ppb).


For pools that have had repeated algae blooms, several per season for several years, your water is choked with dead plant matter - the skeletal remains of previous algae colonies. But not just that, but if your pool water has not been changed in over 5 years, you probably have a high TDS, a buildup of solids in the water which provides food for algae and shelter from chlorine and sunlight.

If you are able to change the water, or even half of the water, you will usually notice much less trouble with algae returning. Replacing the water is not a popular option in many areas of the country affected by drought, but if you can do it, it makes it harder for algae to return. An option to water replacement is truck mounted pool water recycling, popular in areas plagued by hard water, especially in arid parts of the US. They pump your water through reverse osmosis filters and softeners and balancers, and return it to the pool as fresh as rain water.


Rough plaster pool surfaces and etched tile grout gives a nice hideaway for algae to hunker down during algae treatment, sacrificing their outer parts to protect the core, the buried root of algae, deep in a tiny crevice. Insidious, right?

In most cases, replastering the pool is not necessary, but rather a lot of work with the pool brush. A chlorine wash of the pool is a popular treatment for rough pools with green and yellow recurrent algae blooms. A chlorine wash is just like an acid wash, except that one pours bleach over the walls and floor (of an empty pool), scrubbing and rinsing thoroughly. 


  • Algae Infected Swim Suits, Pool Noodles, Pool Lounges
  • Low Chlorine and High pH levels
  • Debris Wash-In from Heavy Storms


~ To keep pool algae from returning, you may have to take some drastic steps, like draining the pool and chlorine washing, and replacing the filter media (sand, de grids or cartridges). And of course, you need to be hyper-vigilant about water balance and chlorine levels, and keep the water over-sanitized and over-filtered.

One more tip: Adding a supplemental sanitizer, like Nature 2 minerals or an Ozonator, is one more way to stop pool algae from coming back. At the very least, faithfully use a good quality (e.g. expensive) algaecide, and add a weekly maintenance dosage.


Thanks for Reading!

Mark Garcia