Why is my pool cloudy, and how can I fix it?

“Why is my pool cloudy?” is probably one of the the most common questions I hear from pool owners – for a very good reason! Unlike green water, which has exactly two causes (algae and green additives), a cloudy pool has a number of causes, and it’s not always straight forward to determine which one applies to you.

If you have a hazy, milky, or cloudy pool, then the simple but very unhelpful answer is that you have very small particles of something in your pool, reflecting back the light. The trick is to try and work out what it is, and how to get rid of it.


Mechanical Issues

The simplest cause of cloudy water, and usually the easiest to fix, is that your pool is just not getting enough filtration. Try looking at the following mechanical issues to see if one applies:

  • Filter not running long enough. Every litre of your pool needs to pass through the filter at least twice per day. Depending on the size of your pool, the size and speed of your pump, and how many bathers you have, the time your filter needs to be running could vary from 2-16 hours per day.
  • Filter not working. Make sure your filter is both working and switched on! Make sure your filter is clean and the filter basket is clear. For sand filters, make sure you backwash until the water runs clear. For cartridge filters, make sure that the running pressure is not too high – and either clean or replace filters as appropriate.
  • Filter not appropriate. Filter and pump size can have big effects on your pool. Too small, and they just can’t do the job. Too large, and it’s possible that water could be pumped through so fast that the filter becomes ineffective.
  • Poor water flow is not enough. Check your pool valves to make sure they are not closed when they should be open, or open when they should be closed. Check the skimmer basket to make sure it’s not blocked with leaves or toys, or in the case of my parents pool, a very wet and angry possum.


Swimmer Issues

Both animal and people use of the pool can contribute to cloudy water. Fortunately, as long as you have the appropriate amount of free chlorine available in your pool after a swimming session, this problem will fix itself. Chlorine takes care of all the sunscreen, body oils, skin cells, hair, and even urine by a chemical reaction and the formation of Chloramines. Fun fact: That “chlorine smell” in a pool is not actually caused by too much chlorine, but by too little! It’s the chloramines making that smell. Just make sure that you have enough chlorine in the pool, and the chloramines will be broken down quickly by UV light – something we have plenty of for free here in Australia!


Chemistry Issues

The balance of chemicals in your pool water can make a huge difference to the cloudiness of your pool. Most chemical problems result in particles too small to be taken out by the filter.

  • Chlorine is too low. Even without swimmers, your pool chlorine is used up when dealing with the everyday dust, pollen, leaves etc., that can fall into the pool. UV also reduces the amount of chlorine in the pool, and inadequate levels of free chlorine can result in an algae bloom. The first visible sign of an algae bloom? Cloudy water.
  • pH is too high. Having your pH levels too high can cause multiple problems. Firstly, at pH >7.8, pool water can’t keep as many salts in solution causing precipitation and cloudiness. Secondly, chlorine isn’t as effective at a high pH, and if you have inadequate amounts of chlorine, this can result in an algae bloom.
  • Too many salts. The overuse of some chemicals, particularly Calcium Hypochlorite can result in cloudy water. Water can hold more salts when the pH is balanced (see above) and when it is cool (see below), but it can’t hold an infinite amount. Testing for both calcium and total alkalinity is important to rule this out. Total alkalinity values of 170 ppm will result in cloudy pools.
  • Water Temperature. Although far less common, water temperature can also play a role in pool water cloudiness. Water temperature above 29ᵒ C means that the pool can hold less calcium salts. Something to bear in mind for those northern Australian states!


Environmental Issues

Even if you have the most well maintained pool in the world, and virtually no one swimming in it, you can still be plagued with cloudiness from sources largely outside of your control.

  • Dust. Probably the one that causes the most frustration. It’s surprising how much of a problem even a small patch of bare dirt near a pool will cause for cloudiness. You can obviously take care of dirt patches in your own yard, but what about the construction site down the street? The general amount of dust from a rural or drought stricken environment? Coarse dust can usually be taken care of by the pool filter, but fine dust will often pass straight through.
  • Pollen. Like dust, this can cause a considerable amount of frustration. Especially given that many pollen particles may well be too too fine for your pool system to filter out.
  • Dead algae. You’ve just treated your pool for an algae infection but the cloudiness is still there? If you’re 100% sure that there is no more growing algae, then the source is likely to be dead algae. Again, often too fine to filter.


How do I remove particles too small for my filter?

You have two choices for dealing with tiny particles. The first is to turn off your filter, let everything settle to the bottom of the pool, and then vacuum to waste. The second is to do the same thing, but use a flocculant which may speed up the settling process. I generally recommend the first option, because adding flocculants can cause a wide range of problems. However, if you have an impending pool party, and need the pool to sparkle, try the flocculant. Just be incredibly careful to follow the directions.


So, with all of the above information, can you work out what issues are causing your cloudy pool?