Acid Neutralizer Media Comparison

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

The media inside your acid neutralizer determines how effectively it corrects your pH. Here's how to choose between calcite, Corosex (FloMag), or a blend of both, based on your water test results.

Want the full picture first? Start with our Complete Acid Neutralizer Guide. Ready to buy? See our Best Acid Neutralizer Buyer's Guide.

Side-by-Side Comparison

Calcite: How It Works

Calcite is crushed calcium carbonate, the same mineral found in limestone and marble. When acidic water flows through a bed of calcite, the acid dissolves small amounts of the calcium carbonate, releasing calcium and bicarbonate ions into the water. This chemical reaction raises the pH naturally. If you are new to the media itself, our explainer on what a calcite water filter is covers the basics in a short video.

For a deeper look at the chemistry behind calcite neutralization, see our guide to how acid neutralizers work.

The beauty of calcite is that it's self-limiting. As the water approaches neutral pH (7.0), the dissolving reaction slows down and eventually stops. This means calcite is almost impossible to overcorrect with. Your water will typically settle between 7.0 and 7.5 pH, which is exactly where you want it.

The tradeoff: calcite adds calcium to your water. If you already have moderately hard water, the additional calcium from a calcite neutralizer will push your hardness higher. That's why we recommend pairing an acid neutralizer with a water softener downstream to handle the added hardness.

When calcite works best

• pH between 6.0 and 6.9
• Low to moderate water usage (1-4 bathrooms)
• Homeowners who want the simplest, lowest-maintenance solution
• Situations where gentle, predictable pH correction is sufficient

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+). Water that's too alkaline is just as damaging as water that's too acidic. It can cause scale buildup, taste issues, and its own set of plumbing problems.

FloMag adds magnesium to the water instead of calcium. Magnesium also contributes to water hardness, so a calcite + FloMag mix produces slightly harder water than calcite alone, though at 2-3 pounds per bag the difference is small. Either way, pairing the neutralizer with a downstream water softener handles the added hardness.

When FloMag is necessary

• pH between 5.5 and 5.9 with a 1.5 cubic foot (10x54) tank (the smaller bed can't keep up on calcite alone)
• High water demand homes where contact time is limited
• Water with pH that drops below 6.0 seasonally (common in spring after heavy rain)

Blending Ratios: Getting It Right

This is where most people (and some water treatment companies) make mistakes. The amount of FloMag/Corosex you add to the calcite is critical: 2 to 3 pounds of FloMag per 50-lb bag of calcite, and never more.

A warning about 50/50 blends: We've seen some online retailers selling small acid neutralizer tanks with a 50/50 calcite-to-Corosex ratio. This is wrong. At 50/50, the pH will shoot well above 8.0 and become alkaline, which can cause the same types of corrosion and scale damage you were trying to prevent. Alkaline water is aggressive in its own way, especially on rubber seals and water heater elements. If you bought a system pre-loaded with a 50/50 blend, drain it and reload with the correct mix.

How to blend properly

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. Instead:

1. Use a clean storage bin. Dump in one bag of calcite, then measure out 2 to 3 pounds of FloMag.
2. Mix the two thoroughly in the bin.
3. Add the mixed media to the tank.
4. Repeat the process for every bag that comes with the unit, so the mix forms even layers throughout the tank.
5. Run water through the system and test the output pH after 24 hours. Target 7.0 to 7.5 during peak usage. If it reads above 8.0, you used too much FloMag.

If we gave you an exact mixture for your water over the phone, follow it. Leftover FloMag keeps indefinitely as long as you store it somewhere dry.

When to Use Calcite Only

Calcite-only systems are the right choice for the majority of homeowners. Here are the scenarios where straight calcite is all you need:

• pH 6.0 to 6.9 in any tank size: This is the most common range for residential well water. Calcite handles it reliably and consistently.
• pH 5.5 to 5.9 with a 2.5 cubic foot (13x54) tank: The larger media bed provides enough contact time for calcite alone to correct the pH, no FloMag needed.
• You want the simplest possible maintenance: Pour in calcite through the dome hole once or twice a year. No measuring, no blending ratios, no pH monitoring after the initial setup.
• You're on a budget: Calcite-only systems cost $200 less than Calcite + FloMag systems, and the calcite media itself ($145 per bag) is less expensive than FloMag ($225 per bag).
• You have hard water concerns: Calcite adds less hardness to the water than a FloMag blend. If your water is already on the harder side, minimizing additional mineral content is better.

For system recommendations, see our best acid neutralizer buyer's guide.

When to Use a Calcite + FloMag Blend

Add FloMag to the mix only when calcite alone isn't enough:

• pH below 6.0 with a 1.5 cubic foot (10x54) tank: The smaller media bed means less contact time, so acidic water passes through faster than the calcite can dissolve. FloMag's faster reaction rate compensates for this.
• High peak water demand: If you have a large home (4+ bathrooms) and everyone showers in the morning, the high flow rate reduces contact time with the calcite. A small amount of FloMag ensures adequate correction even at peak flow.
• Seasonal pH drops: Some wells see pH dip below 6.0 in spring when heavy rain dilutes groundwater. A light FloMag mix (2-3 lbs per bag of calcite) provides a buffer for these fluctuations without overcorrecting during drier months.

When purchasing a new system, you can buy acid neutralizers pre-loaded with a calcite + FloMag blend. We mix the ratio before shipping so it's ready to install.

Common Mistakes to Avoid

1. Using too much FloMag

This is the most common mistake. More FloMag does not mean better pH correction. It means overcorrection. We once sold a system to a contractor with instructions to mix 2 to 3 pounds of FloMag per bag of calcite. He dumped in the entire 50-lb bag. The pH shot up to 11, and he had to empty the tank, buy all new media, and start over. Stick to 2-3 pounds per bag of calcite.

2. Using FloMag alone

Never load a tank with 100% FloMag. The reaction is too aggressive and too fast. There's no self-limiting mechanism like calcite has. Your water will become dangerously alkaline.

3. Pouring FloMag in after the calcite

Never dump 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 new one. Mix each bag of calcite with its 2-3 pounds of FloMag in a storage bin first, then add the mixed media to the tank. Repeating that bag-by-bag creates even layers throughout the bed, so no zone is overcorrected while another stays acidic.

4. Not retesting after adding FloMag

Whenever you change the media ratio (adding FloMag for the first time, or changing the ratio during a refill), test your output water pH after 24 hours. Adjust the blend if the output pH is above 8.0 or below 7.0.

5. Using the wrong media for your pH level

If your pH is 6.5 and you're using a FloMag blend, you're adding unnecessary cost and complexity. If your pH is 5.5 and you're running calcite only in a 1.5 cubic foot tank, you're not getting adequate correction. Match the media to your water test and your tank size.

Refilling and Maintenance

The maintenance for either media is the same process: open the dome hole on top of the tank and pour in media. The difference is what you pour in and how often.

Calcite only systems

• Refill when the media level drops 4 to 6 inches below the dome opening (check visually by opening the dome hole)
• Typically every 18 to 36 months, depending on tank size, starting pH, and water usage (smaller tanks and very low pH refill sooner)
• Use 1 to 3 bags of 50-lb calcite per refill depending on tank size and how far the level dropped

Calcite + FloMag blend systems

• FloMag dissolves a bit faster than calcite, which is exactly why each bag gets bin-mixed before loading: even layers throughout the tank mean you don't have to service it more often than a calcite-only system
• Refill on the same 18 to 36 month schedule as a calcite-only system. FloMag is only mixed in at 2 to 3 pounds per bag, so it doesn't change how often you top off
• Keep the same mix when refilling: 2 to 3 pounds of FloMag per 50-lb bag of calcite, mixed in a bin before it goes in the tank
• Store leftover FloMag in a dry place; it doesn't go bad, and one bag covers many future refills

For the full maintenance walkthrough, see our servicing your acid neutralizer guide.

Cost Comparison

For a complete pricing breakdown including installation and package deals, see our acid neutralizer cost guide.