Why does my pool turn green when it rains?

After several years in drought, mother nature seems to have taken much of Australia to the other extreme! Many parts of the country are experiencing incredible amounts of rainfall – I think we had 100mm here in parts of  Brisbane just yesterday!

Add to that the gorgeous Qld sunshine and 30 degree heat, and I expect quite a few pool owners looked out their back doors this morning to a lime green pool…

A green pool is not an uncommon problem.  If your free chlorine was on the low side of normal before the rain, add an extra couple of cm of water in the pool, add a little sunshine, warm the pool up with your solar blanket, and presto – you’ve got increased chlorine demand and the perfect environment for new algae to blossom. This can happen in just a few hours and when you have increased chlorine demand, then adding the normal amount of chlorine (like most automatic and salt chlorinators do) isn’t enough. You need an increase in the amount of chlorine than normal, to meet the increased chlorine demand.

So despite what you might find on ‘yahoo answers’, a green pool after heavy rain is not caused by lightning, a lack of nitrogen in your pool, or just the rain washing pollen into your filter. It is simply a case of the extra water in the pool throwing your chemical balance out by increasing the chlorine demand, and therefore allowing algae to grow.

So what do you do? If you can, test the water yourself to confirm you chlorine level is too low. Then add the recommended amount of chlorine based on the size of you pool, the existing chlorine level and the recommended dosage on the chlorine container.  Otherwise, it’s time to head to your pool shop with a water sample and they will be able to test and confirm how much to add for you. Recommended methods of correcting your imbalance will vary depending on your particular pool finish, but you need to get your free chlorine back to the correct levels, as soon as possible.  If you have a pool cover, it is important to remove this first – and don’t replace it until your water balance is back within the Australian Standard. This will be a minimum of 48 hours if you are shocking your pool, but use your test strips to check.

As a preventative, during periods of heavy, regular rainfall, it’s a really good idea to check your free chlorine levels every day – and don’t let them fall below the Australian Standard.  So keep your water balanced, stop your pool water level from getting too high and you should enjoy a blue pool, no matter how much rain mother nature dumps on us.




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