Why did the copper green return?


LifeTime Supporter
May 12, 2014
Long Island, NY
Hi TFP community, I have a chemistry situation I am curious about.

I have completely new fill water (never been balanced) after a liner change, city water. A couple weeks ago, chlorine on its own was making the water green (not algae). The previous water never behaved this way. I expect that my city water must have copper in it, not uncommon from what I've read. I don't know where else the copper could be coming from since i have no heater (but wish I did). After research I figured to add some ProTeam metal magic and it solved the green problem (though I didn't actually check the water's pH).

I'm finally getting around to the chemicals (1 week later). I put in fresh chlorine yesterday (to FC=5, still no CYA though) and discovered the pH was high (8.0) and alkalinity only a little low. I added pH reducer (Sodium Bisulfite), but carelessly overshot (pH now just below 7). Everything sat this way fine overnight, no green.
This morning I added the baking soda to increase the alkalinity. After an hour, green started creeping back, and several hours later it was quite green. I then added pH increaser (Calcium carbonate) to get pH to normal (7.4) and the green remained.

I guess the green now means I need to add another round of metal chelator, but am somewhat surprised that the city water seems to be so full of metal. Does this sound unusual?
On the chemistry side: Is the metal magic is no longer effective after the pH decrease (from 8.0 to 7)? Or does it simply dissipate after a week at pH 8.0? I am wondering why did my green come back right after adding the baking soda (while the pH was at 7)? There was obvioulsy still chlorine there, but it wasn't reacting until the increase in alkalinity caused it to suddenly do so. Is the effectiveness of the metal magic very dependent on the alkalinity? It sounds weird, but that is what happened. Is there a different metal chelator that is less finicky than metal magic?
thanks for advice/observations


TFP Expert
Platinum Supporter
May 23, 2015
Tucson, AZ
It’s iron, not copper. Iron in water creates a yellowish hue that combines with the normal blue color of water to give green. I grew up on LI, your fill water has iron in it (my parents fought with iron stains all their lives in our home growing up). Adding chlorine to water at high pH precipitates iron oxide. Given enough time, your filter will eventually remove it. Using sequestering agents can help, but that’s not a one time deal - once in sequestrants, always on sequestrants.
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