Green Pool, But No Phosphates?

When a pool turns green it could be for a few reasons. But if it's green from algae, there is a proven clean-up procedure pool pros can use. Green pools, after all, are not uncommon. But if you have ever tested for phosphates in a green pool, chances are the levels were very low, if not zero. How is that even possible?

Covered in this article:

How to know if there are phosphates in your pool

We are often asked how to know if you have phosphates in your pool. Normally, we say to test for them, but we also know the limitations of phosphate test kits. For one thing, most test kits only measure orthophosphates, not organophosphates or condensed phosphates (like polyphosphates or metaphosphates). So you could have more phosphates in your water than you tested

So here are two reliable ways of knowing if phosphates are in your water: the red cap test, and the presence of algae.

The red cap test

The most reliable way of knowing if you have phosphates, in our opinion, is the red cap test.

PR-10,000 red cap test, small cloud showing phosphates are present in the pool
The cloud from a PR-10,000 red cap test after 30 seconds.

It's simple: if PR-10,000 clouds the water, there are phosphates in the water. And you're watching phosphates being converted into a solid and precipitating out of solution (into a cloud of fine white dust). This is an irreversible reaction, so the phosphate fallout cannot be re-dissolved into the water. Those phosphates are removed for good. Then you just need to vacuum them out and/or clean the filters to remove the dust from the pool.

Is the red cap test accurate for testing levels? No, because that's not the point. It's more of a yes-or-no test to see if phosphates are in the water. But it is 100% reliable that it will react with phosphates if they're in the water.

Where there are algae, there are phosphates

According to every reputable source on the matter1, phosphates are essential for microorganisms (like algae) to grow and reproduce.

And while phosphates are essential to algae, removing phosphates from water does not kill algae. Only sanitizers–like chlorine–and EPA-registered algaecides will kill algae. No Orenda products (nor any phosphate removers) kill algae. Phosphate removers are only a supplement to clean up after chlorine does its job.

Related: Phosphate Removal (Pillar 3)

Testing Zero Phosphates

Now that we know that algae prove there are at least some phosphates in the water, how come it's possible (and common) to test for phosphates in a green pool and read zero? The answer is simpler than you might think: phosphates are held within algae's cell walls, and therefore not in solution. Phosphates cannot be picked up by a test kit if they're not in solution.

But when algae die, their phosphates are released back into the water.

PR-10,000 in a green pool clouding, PR-10000, orenda phosphate remover, phosphates, phosphates in algae, green pool, green pool clean up, orenda green poolWithin seconds, PR-10,000 reacts with phosphates in this swamp-green pool. This pool tested zero phosphates minutes earlier prior to the chlorine shock that preceded this PR-10,000 dose

Green pools can have very high phosphates AND pH

Another thing that can exceed most test kits is pH. Green pools that are taken over by algae have an extraordinarily high pH because algae have consumed so much carbon dioxide. Less CO2 in solution means the pH goes up. See our video from the field explaining more:


We only bring this up because like phosphates, pH readings can be distorted by algae. Once algae die and the CO2 is released back into the water, the pH should decrease. But in the case of a green pool, you would have already lowered the pH before adding the chlorine shock anyway, so it's a moot point.


Pools that turn green from algae are guaranteed to have phosphates in them. But in many cases, most (if not all) of the phosphates are contained within the cell walls of the algae, and therefore unreadable on the test kit. But once algae die, their phosphates, CO2, and other nutrients are released back into the water.

Phosphate test kits have other limitations too. Namely, most of them only read orthophosphates, despite there being many different types of phosphates that can be in pools. So for a reliable way of knowing if phosphates are present in your pool (besides visible algae), try adding just a capful of PR-10,000 and see if it clouds the water. The cloud is proof that phosphates are in the water.


1  Every textbook and peer-reviewed article we have found on the matter confirms that phosphates, nitrates and dissolved CO2 are nutrients for algae. It is common knowledge in the pool industry. But we are careful in how we phrase these facts because we are a manufacturer of phosphate remover products. We must be clear that phosphate removers do not kill algae, even though they remove phosphates.

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