Calcite vs. Corosex (FloMag): Which Acid Neutralizer Media Do You Need?

Calcite or Corosex? For most wells the answer is plain calcite. Corosex, which we sell as FloMag, is a pH booster you mix into the calcite only when your pH is too low for the size tank you have. Your pH number and your tank size decide it, not your flow rate or how many bathrooms you have.

Caller "My pH is really low, like, I think it's 6.0 ... So what is this FloMag media filter kit?"

Aidan "That's only if your pH is below 5.5. That's a pH booster you add to the calcite. You don't need that."

A Virginia caller in July, pricing a 2.5 cubic foot tank for a house with five bathrooms. This page is the long version of that answer: which pH needs the booster, in which size tank.

Calcite vs Corosex side by side

Calcite does the work in every tank; Corosex is only ever an additive mixed into the calcite. Here is how the two compare.

What calcite does in the tank

Calcite is crushed calcium carbonate, the same mineral found in limestone and marble. When acidic water flows through the tank, the acid slowly dissolves a little of the calcite, and that raises the pH. If you want the full story on the mineral itself, I cover it in what a calcite filter is.

Calcite is self-limiting: once the water reaches neutral, it stops dissolving. On calcite alone the water comes out anywhere from 7.0 to 7.5, which is where you want it. That is why a tank of plain calcite is so hard to get wrong.

The trade-off is hardness. The calcium that raises your pH also adds a few grains of hardness, typically 3 to 6 grains per gallon. At 7 grains and above I recommend a softener after the tank; a neutralizer with a softener covers how the two work together.

Corosex (FloMag): How It Works

Corosex is a brand name for magnesium oxide media. We sell it as FloMag, which is the same material. When acidic water contacts magnesium oxide, the pH correction reaction happens roughly 5 times faster than with calcite.

That power is exactly why Corosex/FloMag is never used alone. If you filled an acid neutralizer tank with 100% Corosex, the water coming out would be highly alkaline (pH 9 to 10+). A contractor I sold a system to years ago dumped a whole 50 lb bag of FloMag in instead of 2 to 3 pounds per bag of calcite; the pH shot up to 11, and he had to dump it all out and buy all new media.

Corosex is Clack's name for it and FloMag is the name on the bag we ship; it is the same thing under two names. It is a natural mineral, certified to NSF/ANSI 60 for pH adjustment of drinking water, not a chemical feed. Mixed in at a few pounds per bag, it makes the water a little harder than plain calcite, but not much more.

When you need FloMag with calcite

Your pH number and your tank size decide whether you need FloMag; nothing else does. Find your tank size and your pH in the table.

pH 6.0 to 6.9 (mildly acidic): Calcite only, in any tank size. That covers most people: of the acidic wells my customers have had lab-tested, about four in five were 6.0 or higher. The Virginia caller above was at 6.0 in a 2.5, five bathrooms and all, and did not need the booster.

A calcite and FloMag blend handles pH into the 4s. Below 5.0 the pounds per bag go up with how acidic the water is, so I set the mixture by phone from your water test; the lowest well we have treated was 3.67. No chemical feed pump is needed for any of it.

Two things do not change the answer. Flow rate doesn't matter when you're using a non-backwashing tank, and more bathrooms do not mean you need FloMag; a bigger tank gives the calcite more contact time, which is why a 2.5 handles half a pH point lower than a 1.5. If your pH is under 6.0 and a 13 inch tank fits your space, the 2.5 lets you skip FloMag down to 5.5. Which size fits your house is on the sizing guide.

No pH number yet? Certified lab test, $199

Your pH decides what you need

One certified lab test, mailed to you, run by an accredited lab, gives you the pH this table keys off. If you already have a lab report, skip this.

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Have your pH and tank size?

Text me your pH number

Send your pH and your tank size. I'll tell you whether you need FloMag and, if you do, how many pounds per bag.

Text Aidan443-277-2204 Call800-460-5810Aidan

8 AM to 5 PM Eastern, seven days a week. A real person, not a call center.

Never pour FloMag in unmixed

Never pour FloMag into the tank on top of the calcite. It can harden into a solid brick, ruin the tank, and force you to start over with a brand-new tank. A Delaware owner who scooped calcite and FloMag in one after the other found a block 12 to 15 inches thick near the top of her tank when she opened it for a refill.

The right way is one bag of calcite in a storage bin, the FloMag measured in, mixed by hand, then into the tank, and the same for every bag. That builds even layers through the whole tank. Step by step, with the pounds for your pH, is in how to mix FloMag with calcite.

Be careful with blends sold as a fixed percentage (80/20, 90/10, 50/50). A percentage skips the question that matters, which is your pH. A buyer at pH 5.4 in 2020 filled his tank with a 10% FloMag mix and the water went to 10+. When I give you a mixture on the phone, follow that one.

Does FloMag change the refill schedule?

No: a calcite and FloMag tank gets refilled on the same schedule as a plain calcite tank. FloMag dissolves a little quicker, and mixing it into every bag spreads it evenly so you don't have to service it more often. The schedule depends on the tank size.

Those are averages; very low pH and heavy water use shorten them. Check the level once a year with a flashlight against the back of the tank, and add a bag when it is about halfway down. On a blended tank, mix the same pounds of FloMag into each new bag of calcite before it goes in. Leftover FloMag doesn't go bad if you keep it dry, and at a few pounds per refill one bag lasts for years. The full routine is in how to add calcite to the tank.

What the FloMag option costs

The FloMag option adds $200 to any size tank. Prices below include shipping.

With the FloMag option, the FloMag arrives as its own bag, blue and white, next to the calcite bags. Every tank ships empty: the calcite and the FloMag are mixed and loaded at install. Drain tubing is not needed on a non-backwashing tank. We recommend installation by a licensed plumber, and I support your plumber by phone at 800-460-5810.

After that, refills cost the calcite, $145 a bag; the FloMag comes out of the leftover bag. If you ever need more, FloMag only comes one way, a 50 lb bag for $225. Every other price, backwashing tanks included, is in the acid neutralizer cost guide.

What buyers wrote about each option

These are verified purchases, some on plain calcite and some on the blend.

Frequently asked questions