Calcite vs. Corosex (FloMag): Which Acid Neutralizer Media Do You Need?
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.
Related Acid Neutralizer Guides
Quick verdict: which media do you need?
- pH 6.0 to 6.9 (mildly acidic): Calcite only. This covers the majority of residential well water. Calcite dissolves slowly, raises pH gently, and is easy to maintain.
- pH 5.5 to 5.9 (moderately acidic): Calcite + FloMag (Corosex) blend at a 90/10 ratio. The FloMag boosts pH correction without overreacting.
- pH below 5.5 (very acidic): Call us at 800-460-5810. At these levels, the media ratio needs careful calibration, and a chemical feed (soda ash) system may be a better option.
If you haven't tested your water yet, start there. A basic well water pH test from a local lab costs $30 to $75 and tells you exactly which media you need.
Which media does your acid neutralizer need?
Enter your pH level and we'll tell you exactly what to use.
In this guide:
Side-by-Side Comparison
| Calcite | Corosex (FloMag) | |
|---|---|---|
| Chemical name | Calcium carbonate (CaCO₃) | Magnesium oxide (MgO) |
| pH correction speed | Slow, gradual | ~5x faster than calcite |
| Effective pH range | 6.0 to 6.9 | Below 6.0 (always blended with calcite) |
| Mineral added to water | Calcium (increases hardness) | Magnesium (increases hardness more aggressively) |
| Dissolve rate | Slow, self-limiting (won't overcorrect) | Fast, aggressive (can overcorrect if not blended properly) |
| Used alone? | Yes, in most residential systems | Never. Always blended with calcite at 10-20% ratio |
| Cost per 50-lb bag | $145 | $225 |
| Refill frequency | Every 6-12 months | FloMag dissolves faster; blend requires more frequent topping off |
| Risk of overcorrection | Very low | High if ratio is wrong |
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.
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, and more aggressively than the calcium from calcite. This means a system with FloMag will produce harder water than a calcite-only system, which reinforces the need for a downstream water softener.
When FloMag is necessary
- pH between 5.5 and 5.9 (calcite alone can't keep up)
- 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 ratio of calcite to FloMag/Corosex is critical.
| Your pH | Recommended Ratio | What It Means |
|---|---|---|
| 6.0 - 6.9 | 100% calcite | No FloMag needed. Calcite handles this range on its own. |
| 5.5 - 5.9 | 90% calcite / 10% FloMag | The standard blend. Provides the extra correction needed without overcorrecting. |
| 5.0 - 5.4 | 80% calcite / 20% FloMag | Maximum ratio we recommend. Used only for confirmed very low pH. Requires careful monitoring. |
| Below 5.0 | Consider chemical feed (soda ash) | At this level, even an 80/20 blend may not provide consistent correction. A soda ash injection system is often more reliable. |
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 reblend at 90/10.
How to blend properly
When refilling your acid neutralizer with a calcite + FloMag blend:
- Measure your calcite and FloMag separately before adding to the tank.
- Pour alternating layers (some calcite, some FloMag, more calcite) to ensure even mixing.
- Run water through the system and test the output pH after 24 hours.
- Target output pH of 7.0 to 7.5 during peak water usage.
- If the output pH is above 8.0, you have too much FloMag. Add more calcite to dilute.
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: This is the most common range for well water in the Mid-Atlantic region. Calcite handles it reliably and consistently.
- 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: At this level, acidic water passes through the calcite bed 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 90/10 blend 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. A 50/50 blend will push your pH to 9.0 or higher, which causes scale buildup, damages rubber components, and creates a metallic taste. Stick to 90/10 or 80/20 maximum.
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. Not mixing the blend thoroughly
If you dump all the FloMag in at once on top of the calcite, the water contacting the FloMag layer first will be overcorrected while the water below remains acidic. Layer it: pour some calcite, add a portion of FloMag, more calcite, more FloMag. This distributes the aggressive media evenly throughout the bed.
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 using calcite only, you're not getting adequate correction. Match the media to your water test.
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 6 to 12 months for average usage
- 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 faster than calcite, so blended systems need slightly more frequent refills
- Typically every 4 to 8 months for average usage
- Maintain the original ratio when refilling: if your system uses 90/10, add 90% calcite and 10% FloMag
- Layer the media as you add it to maintain even distribution
For the full maintenance walkthrough, see our servicing your acid neutralizer guide.
Cost Comparison
| Calcite Only | Calcite + FloMag Blend | |
|---|---|---|
| System price (2.5 CF) | $1,495 | $1,695 |
| Media cost per bag | $145 (calcite) | $145 (calcite) + $225 (FloMag) |
| Annual media cost | $290-$435 | $350-$550 (more frequent refills + FloMag cost) |
| 5-year total | $2,945-$3,670 | $3,445-$4,445 |
For a complete pricing breakdown including installation and package deals, see our acid neutralizer cost guide.
Frequently Asked Questions
What is the correct ratio of calcite to Corosex/FloMag?
For most applications: 90% calcite to 10% FloMag. For very low pH (5.0-5.4), you can go up to 80/20, but never exceed that. A 50/50 blend is too aggressive and will overcorrect your pH above 8.0, causing scale and corrosion problems.
Can I add FloMag to my existing calcite-only system?
Yes. If your pH is testing lower than expected and your calcite system isn't fully correcting it, you can add FloMag during your next refill. Calculate 10% of your total media volume as FloMag and mix it with the calcite as you pour it in. Test the output pH after 24 hours and adjust if needed.
Is Corosex the same thing as FloMag?
Yes. Corosex is a brand name originally trademarked by Clack Corporation. FloMag is the same magnesium oxide media sold under a different brand name. The chemical composition and performance are identical. We sell it as FloMag ($225 per 50-lb bag).
How much does a 50-lb bag of calcite cost?
A 50-lb bag of calcite costs $145 from Mid Atlantic Water. Most homes use 2-3 bags per year for ongoing refills. We also sell FloMag for $225 per 50-lb bag, though you need far less of it since it's blended at a 10-20% ratio.
Does calcite make my water hard?
Yes. As calcite dissolves to raise your pH, it releases calcium into the water, which increases hardness. This is a normal part of how the system works. Most homes with an acid neutralizer also install a water softener downstream to remove the added calcium and prevent scale buildup.
What pH is too low for calcite to handle alone?
Below 6.0. Calcite alone becomes less effective at pH levels below 6.0 because the water is too acidic for the slow dissolving reaction to fully correct the pH before the water exits the tank. At pH 5.5-5.9, add a 10% FloMag blend. Below 5.5, consider a chemical feed system.
How do I know when my acid neutralizer needs more media?
Open the dome hole on top of the tank and look inside. If the media level has dropped 4-6 inches below the opening, it's time to add more. You can also test your output water: if the pH is starting to drop below 7.0, the media bed may be getting low. Most homeowners check every 6 months and refill as needed. See our servicing guide for the full process.