Green Water That Is Not Algae

The five-minute test

1. Fill a clean white bucket with pool water.
2. Put it somewhere dark and leave it for 24 hours.
3. Algae needs light and will settle or fade; a metal tint will not change.
4. For a faster read: add a small amount of chlorine to the bucket. Algae water clears slightly. Metal water goes a deeper green almost immediately, because you have just oxidized the metal.
5. Look at the pool surfaces too. Algae is slimy under your hand and clings to walls. A metals problem leaves surfaces clean.

Where the metal came from

The most common cause is a heater and low pH

If your pool has a heater with copper heat-exchanger elements and your pH has been running low, the water has been quietly dissolving the heater. Low pH is corrosive, and the copper it strips ends up in the water column. Then someone shocks the pool, the copper oxidizes, and the water goes green overnight.

That sequence is worth recognizing because the durable fix is not a chemical. It is getting pH and alkalinity back into range and keeping them there - how to balance pool water - so the water stops attacking the equipment.

What to do about it

• **Stop shocking.** Every dose of chlorine oxidizes more metal and deepens the color.
• **Get pH into range first.** Metals precipitate and stain as pH rises, so correcting alkalinity before pH matters here more than usual.
• **Use a metal sequestrant, not a shock.** Sequestrants bind dissolved metal so it stays in solution rather than staining, and they need re-dosing periodically because they degrade.
• **Filter after sequestering.** Some metal will come out as fine particulate and the filter is what removes it. A clarifier helps here.
• **Fix the source.** If it is well water, use a hose pre-filter next time you top up. If it is a heater, get the heater looked at. If it is a copper algaecide, stop using it - see best pool algaecide for copper-free options.

If it is a mineral cartridge doing it

Mineral sanitizer units and copper-based algaecide cartridges work precisely because they put copper into the water. They are effective, and on a pool whose pH drifts up they are also a staining risk that the packaging is not obliged to explain.

A pool running a copper mineral unit needs pH watched more carefully than one that is not, because copper comes out of solution as pH rises. If your pH is hard to hold, that is a reason to choose a copper-free polyquat instead.

Well water is its own case

Filling from a well introduces iron and sometimes manganese directly. The tell is timing: the pool discolours after a top-up rather than after a hot week. A hose-end pre-filter costs very little and removes most of it before it reaches the pool, which is far easier than removing it afterwards.

If the water looks clear at the shallow end and brown looking down the deep end, that is iron in suspension rather than a stain on the floor. Sequester, then filter.