If you just gave your pool a shock treatment and the water tinted green, teal or even reddish within hours, instead of clearing up like you expected, it's most likely not that algae appeared out of nowhere. It's a dissolved metal, usually copper or iron, that was invisible in the water and that the shock oxidizer itself just precipitated out.
Why chlorine “reveals” the metal
Why chlorine "reveals" the metal instead of clearing the water
Fill water can carry tiny amounts of dissolved copper or iron without you ever noticing, they come from an old pipe, a well, a heat exchanger with copper parts, or a salt chlorinator cell starting to wear down. While the metal stays dissolved, the water can look perfectly clear. The problem is that shock chlorine is a strong oxidizer: when it contacts those dissolved metals, it oxidizes them into very fine solid particles, and those particles are what color the water. Oxidized copper tends toward blue-green or teal, and oxidized iron toward red, orange or brown, depending on the concentration.
It's a fast reaction: unlike an algae bloom, which takes time to develop, metal staining can show up within minutes or hours of the shock, while the oxidizer is still working in the water.
How to tell it apart from an algae bloom
Both give you green water, but there are clear signs that separate them:
- Timing: metal staining shows up very soon after a recent shock treatment. An algae bloom is usually something you could see coming, somewhat cloudy water, chlorine low for several days, and it doesn't line up with just having applied a strong treatment.
- Color and texture: metal staining tends to be more uniform throughout the water, often with a blue, teal or reddish tint rather than the more "vegetal" green typical of algae. There's no slimy or slippery surface on the walls and floor.
- Response to brushing: with algae, brushing the walls and floor usually loosens visible material and helps the treatment. With metals, brushing changes nothing, the color is in the water, not stuck to a surface.
- Response to a second shock: with algae, another shock treatment usually improves things. With metals, repeating the shock blindly can oxidize even more metal and darken the problem further.
What to do if it's a metal problem
The approach is different from an algae bloom, and comes down to three things:
- Apply a specific metal sequestrant, a product designed to "bind" the dissolved or precipitated metal and stop it from oxidizing further or staining surfaces.
- Keep the filtration running continuously so the filter can pull the already-precipitated particles out of the water.
- Avoid further shock treatments until the water has cleared, and check the source of the metal, fill water, salt chlorinator cell, metal fittings in contact with the water, so it doesn't happen again.
If you're also noticing colored stains on the walls or floor, it's the same family of problem: metals that settled onto a surface instead of staying suspended in the water. We cover that in full in our guide on pool stains and how to remove them.
Frequently asked questions
Why did the water turn green right after adding chlorine, when chlorine is supposed to clear it up?
Because the water already had dissolved metals in it (usually copper or iron) that were invisible while dissolved. Shock chlorine is a strong oxidizer, and it oxidizes those metals all at once, precipitating them out, and those oxide particles are what give the water its green, blue or reddish tint.
How do I know if it's a metal problem and not algae?
Look at the speed and the shade: metal staining appears within minutes or hours of the shock, with a more uniform color (blue-green, teal or reddish) and none of algae's sticky texture on walls and floor. Brushing doesn't change anything, and a second shock treatment usually makes the problem darker instead of fixing it.
Can I just shock the pool again to clear it up?
Not a good idea if you suspect metals. More chlorine oxidizes more metal and can darken the water further or leave stains on the walls. The right move is a metal sequestrant, good filtration, and letting the system remove the particles that have already precipitated.