This phrase is used for two products that do opposite things, which causes real confusion when buying. One is a neutralizing tank filled with calcium carbonate media that dissolves into acidic water to raise its pH, adding hardness. The other is anything sold to deal with calcium carbonate scale, which means removing hardness.
The filter that adds it
A calcite neutralizer is a backwashing tank of crushed calcium carbonate, and acidic water dissolves a small amount as it passes, consuming acidity and raising the pH towards neutral. It is the standard treatment for a private well corroding copper pipework, recognized by blue green staining in sinks and baths and eventually by pinhole leaks. The media is consumed by doing its job, so the bed is topped up once or twice a year rather than left alone.
The filters that deal with it
Calcium carbonate is what scale is made of, so anything sold against limescale is working against it. A softener removes calcium and magnesium outright by ion exchange, which is the only approach that stops new scale entirely. A template assisted crystallization conditioner leaves them in the water and reduces their tendency to deposit on hot surfaces. A polyphosphate cartridge sequesters them so they stay in solution. None of these is a filter in the strict sense.
Why the confusion has consequences
Because the two are sold under overlapping descriptions, a household with hard water can end up buying a neutralizer that makes the water harder still, and a household with acidic water can end up with a softener that does nothing about the corrosion. The way to avoid it is to buy from a test result rather than from a product name: pH tells you whether you need to add, and hardness tells you whether you need to remove.
When you need both, and in what order
This is common on private wells, where water can be both acidic and hard. Neutralizer first, because pH correction belongs early in the train and iron oxidation downstream depends on it, then the softener sized for the hardness the calcite adds rather than the original figure. Getting that order wrong produces a softener working harder than it was specified for and a shorter interval than expected. Our whole house filter guide covers building the train.
Questions people ask about calcium carbonate filter
Does a calcium carbonate filter add or remove hardness?
A calcite neutralizer adds it, by dissolving into acidic water to raise pH. Products sold against limescale remove or sequester it.
How do I know which I need?
From a test. pH tells you whether the water is acidic and needs neutralizing; hardness tells you whether it needs softening.
Can I need both?
Commonly, on wells that are acidic and hard. Fit the neutralizer first, then a softener sized for the hardness it adds.
What maintenance does a calcite tank need?
Topping up the media once or twice a year, since it is consumed. A bed left low lets pH drift down and corrosion resume.