Water filter type
Acid neutralisers
A calcite or magnesia bed that raises the pH of acidic water, which stops it dissolving copper out of the pipes it runs through.
The stain is your plumbing dissolving
Blue-green staining below a tap is copper. Acidic water is slowly dissolving the inside of the copper pipe it travels through, and the colour you can see is the metal that has been carried to the fixture.
That makes this different from most water quality complaints. It is not only about the water. It is about the plumbing being consumed, and left alone it ends in pinhole leaks, usually in the hot lines first because heat accelerates the reaction.
Water below about pH 6.5 is considered corrosive. On a private well this is common in areas with granite bedrock and little natural buffering. On municipal water it is unusual, because utilities adjust pH at the plant precisely to stop this happening in customers' pipes.
How a neutraliser corrects it
The tank holds calcite, which is crushed limestone, sometimes blended with magnesia for water that is very acidic. As water passes through, a small amount of the media dissolves and raises the pH toward neutral.
Because the media is consumed by design, it needs topping up rather than replacing, typically once or twice a year depending on how acidic the water is and how much you use.
Most units backwash periodically to prevent the bed compacting, which needs a drain connection. Simpler cartridge versions exist for low demand but they need attention more often.
The side effect worth planning for
Calcite raises pH by dissolving calcium into the water, which means it also raises hardness. If you already have a softener, it now has more work to do and will regenerate more often.
Where both are needed, order matters. The neutraliser goes first so the softener sees the corrected water, and the softener is sized for the hardness after neutralisation rather than before.
Existing blue-green stains do not lift on their own once the water is corrected. They need cleaning separately. What stops is new staining, and more importantly, further loss of pipe wall.
What it reduces, and under which standard
Each row names the NSF/ANSI standard the category is tested under. A standard describes what the test covers. It does not mean every unit on the market carries that certification, so check the specific model against its listing before you buy.
| Contaminant | Standard | Note |
|---|---|---|
| Low pH, corrosivity | NSF/ANSI 42 |
What it does not do
Every filter has a boundary. These are the things this one will not fix, listed so you can rule it out quickly if your problem is on the list.
- Copper already dissolved into the water upstream of the unit.
- Existing blue-green staining, which needs cleaning separately.
- Bacteria, nitrates, or organic contaminants.
- It also raises hardness slightly as calcite dissolves, which can matter if you already have a softener.
When it fits
- Blue-green staining on sinks, tubs, and fixtures
- A well test shows pH below about 6.5
- Pinhole leaks are appearing in copper pipe
When it does not fit
- pH is already near neutral
- The staining is orange, which is iron and not acidity
- You are on city water, where pH is normally controlled at the plant
Space, services, and upkeep
A tank around 1 foot by 5 feet at the main line, with backwash drain access.
A drain connection is required. It needs no power. This is not a rental option, because it fixes to the building and stays there.
Replacement interval. Calcite media topped up every 6 to 12 months as it dissolves.
Problems this is used for
We recommend this equipment against these confirmed problems. Each page explains how to confirm the problem before spending anything, and when treatment is not warranted.
- Low pH and blue-green stains. Acidic water dissolves copper from the pipes it passes through.
Property types where this comes up
Common questions
Are the blue-green stains harmful?
The stain itself is cosmetic, but it tells you copper is dissolving into your drinking water, and that is worth acting on. It also means the inside of your pipework is being consumed, which ends in pinhole leaks, typically in hot lines first. A laboratory copper test alongside a pH reading establishes how much is being picked up.
Will it make my water hard?
Slightly, yes. The neutraliser works by dissolving calcite, which is a calcium compound, so hardness rises as pH rises. Where you also need softening, the neutraliser goes first and the softener is sized for the hardness after neutralisation rather than before. Designing those two in isolation is a common way to end up with an undersized softener.
How often does the media need topping up?
Typically once or twice a year, because the calcite is consumed by design rather than clogged. How fast depends on how acidic the water is and how much you use. Most units also backwash periodically to stop the bed compacting, which is why a drain connection is usually needed.
Other filter types to compare
If the notes above rule this out, these are the neighbouring options worth reading before you decide.
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