Everything is going swimmingly (pun intended!) and your pool looks crystal clear, you decide to do your regular maintenance tasks and shock the pool to get rid of anything you might have missed and then… Just like what happened in the Rio Olympics your pool water magically turned green!

What happened to your pool water?

While it’s easy to assume that it’s algae that are causing the green water, we must remember that we started off with a crystal clear pool and we just shocked it. The first thing we should always do when we have green pool water is to test our chlorine levels. Yes, we just shocked the pool so we can assume that your chlorine will be at its proper levels. And if this is the case then there are a couple of culprits that we can turn our attention to.

Culprit 1: Heavy Metals in the pool water

Did you just have your pool water topped up from an unknown source? A very likely source is that the water added to your pool contains heavy metals. The only way to determine this is to either get some specialized test strips or bring a water sample over to your local pool shop as your regular test strips don’t typically test for metals in pool water.

Culprit 2: Copper-Based Algaecides

Didn’t have water added to your pool? Then another possible culprit could be the excessive use of copper-based algaecides. While copper-based algaecides are very useful for ensuring that algae growth is prevented, using too much can actually spike up the copper levels in your pool to make it reactive enough to your chlorine to turn your pool water green!

Culprit 3: Low pH + Metal Plumbing

The third possible culprit, if you strictly use in-house water and you’re judicious with your algaecide use is having a combination of constant low pH levels and metal plumbing. What happens is that your water will start to eat away at your metal plumbing and will release heavy metals into your pool, which will react with high levels of chlorine especially when shocking your pool.

Culprit 4: Living in an area with heavy metals in the water supply

If you do live in an area where your water supply naturally has heavy metals then you should already be aware that filling up/topping up your water will increase the heavy metal content in your pool. This is more of a long-term problem you by now you shouldn’t be surprised if your pool water turns green after shocking the pool.

Dealing with green pool water caused by heavy metals

Since it’s a metal issue causing the green water in your pool, then we must address the metal contaminants in our pool and we have a couple of options to go with it.

  • Metal Removing Chemicals - These are chemicals that bind with metal particles, crystalizing them and making them large enough for your filters to deal with. This is an excellent way to deal with one-time problems like when you’re refilling the pool with water from a municipal source. For on-going problems like if your own local water source having heavy metals, either go for a preventative measure (more on that later) or stock up on metal removing chemicals and figure out a regular dosing regimen to constantly keep metals out of your pool.
  • Metal Sequestering Agents - These types of chemicals are added to swimming pools to keep metals in solution. This prevents metals in your pool from reacting with the other pool chemicals like chlorine so that you don’t get your pool water turning green from the reactions. It is worth noting though that sequestering agents don’t actually remove the metals from the water so you’ll have to keep adding sequestering agents on a monthly basis.

Preventing green water caused by heavy metals

Note: The photo above is just a sample of the types of metal-pre filters available in the market

It goes without saying that if we want to prevent green water caused by metals is to prevent metals from entering your pool water in the first place! Now depending on the source of the metal contamination in your pool, the prevention method may be different.

  • Metal contamination coming from external sources - If you’re refilling your pool water with water from a source with high metal content then you should always test it for metal content before dumping it into your pool. If it has a high metal content, then consider using a pre-filter (ask your water supplier about fitting a pre-filter before putting in the water) before filling up your pool. If a pre-filter is out of the question, then the next best thing to do is to add some metal removing chemicals into your pool and allow it to circulate before adding your chlorine to take out as much metal as possible.
  • Metal contamination coming from copper-based algaecides - If this is the source of your metal problem then you have two options. The first option is to reduce the amount of algaecide you use in your pool. The second option is to an algaecide that isn’t copper-based. All you need to do is figure out which is the best for your situation.
  • Metal contamination from metal pipes - If your plumbing is the source of your metal contamination then the best thing you can do is to keep your pH levels from dropping to ensure that it doesn’t eat away at your pipes and depositing metals in your pool water.
  • Metal contamination from local water source - If your local water source has high levels of metals then the only thing you can do is install a pre-filter on your hose to filter out as much metals from the water before it enters your pool. Adding a metal sequestrant on a regular basis on a regular basis goes a long way in preventing green pool water when you’re shocking your pool.

Note: When we’re talking about pre-filtering your water for metals, we’re not talking about those vortex-type pre-filters (although those are amazing to have in your pool as well) but pre-filters designed to remove metals from your pool water without the use of chemicals.


As you can see, green pool water isn’t always caused by algae, it can be caused by the actual things that you use to get rid of algae (chlorine!) So the next time your pool water turns green after shocking, don’t reach for more chlorine thinking that it’s an algae bloom. Test your pool water for metals first to make sure that it isn’t a metal problem!

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Happy swimming :)

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