Acid Neutralizer Media

How to Use FloMag (Corosex) in Your Acid Neutralizer

The step-by-step guide to mixing FloMag with calcite: correct ratios for your pH level, how to add it to the tank, and what happens when you use too much.

Want the full picture? Start with our Complete Acid Neutralizer Guide. Not sure which media you need? See our Calcite vs. Corosex comparison.

What Is FloMag (Corosex)?

FloMag is a granular magnesium oxide media used inside acid neutralizer tanks to raise the pH of acidic well water. It's the same product as Corosex (a brand name from Clack Corporation). The terms are interchangeable.

Here's the key thing to understand: FloMag reacts roughly five times faster than calcite. Where calcite (calcium carbonate) dissolves slowly and gently raises pH, FloMag dissolves aggressively and pushes pH up quickly. That's why you never use it alone. You blend it with calcite so the aggressive reaction is diluted across the entire media bed.

Think of it like seasoning. Calcite is the base ingredient. FloMag is the spice. A little goes a long way, and too much ruins the result.

For a deeper comparison of how the two media work chemically and which one to choose, see our calcite vs. Corosex guide. This article focuses on the practical side: how to actually use FloMag in your system.

When You Need FloMag (and When You Don't)

Most homeowners with acidic well water don't need FloMag at all. Calcite alone handles the majority of residential situations. You only need to add FloMag when your pH is low enough that calcite can't fully correct it by itself.

We tell customers this regularly: one homeowner with a pH of 5.5 was worried about overcorrecting and asked whether to start with FloMag or just calcite. Our advice was to install with calcite first and see where the output landed. Calcite alone got them close to neutral, and they added a small amount of FloMag at their first refill to bring it the rest of the way. No guesswork, no overcorrection.

Correct Mixing Ratios by pH Level

The ratio of FloMag to calcite depends on how acidic your water is. More acidic water needs more FloMag, but you increase gradually, not all at once.

Your target output pH is 7.0 to 8.0 during peak water usage. This range protects your plumbing and appliances from corrosion without creating alkaline problems on the other end. If your output pH is above 8.0, you're using too much FloMag. If it's below 7.0, you need more.

How to Mix and Add FloMag: Step by Step

Whether you're loading a brand new system or refilling an existing one, the mixing process is the same. Do this outside or in a garage if possible; the media is dusty.

What you'll need

• Calcite (50-lb bags): number depends on tank size (3 bags for 1.5 CF tank, 5 bags for 2.5 CF tank)
• FloMag (50-lb bag): one bag is enough for years of refills at standard ratios
• A large plastic storage bin (the kind you get at Walmart or Home Depot)
• A fill funnel (comes with your acid neutralizer)
• A digital kitchen scale or a measuring method for weighing 2 to 5 lbs

The process

1. Weigh out your FloMag. For your first bag of calcite, measure the correct amount of FloMag based on the ratio table above. For a 2.5 cubic foot tank with 5 bags of calcite at a 90/10 ratio, that's 5 lbs of FloMag per bag.
2. Dump one bag of calcite into the storage bin. Pour the entire 50-lb bag into the bin.
3. Add the measured FloMag on top. Spread it across the calcite as evenly as you can.
4. Mix thoroughly. Use your hands (wear gloves) or a shovel to mix the two media together until the FloMag is evenly distributed. FloMag granules are slightly darker and coarser than calcite, so you can see the blend visually. If you can see clumps of FloMag, keep mixing.
5. Pour the blended media into the tank. Use the fill funnel through the fill port on top of the tank. Go slowly to avoid bridging.
6. Repeat for each bag of calcite. Mix and add one bag at a time. This layered approach ensures even distribution throughout the entire media bed.
7. Store the remaining FloMag. Seal the bag tightly and keep it in a dry place. One 50-lb bag of FloMag lasts most homeowners several years of refills.

Adding FloMag to an Existing Calcite-Only System

If you already have an acid neutralizer running with calcite and your output pH is testing below 7.0, you can add FloMag without completely replacing the media. Here's how:

1. Check the media level first. Open the fill port and look inside with a flashlight. If the media level is more than 4 to 6 inches below the opening, you need to add media anyway. This is the perfect time to start blending in FloMag.
2. Mix FloMag with fresh calcite in a bin (same process as above) and pour the blend on top of the existing media through the fill port.
3. Run water for 10 to 15 minutes after adding media to rinse the dust and settle the new media into the bed.
4. Test your output pH after 24 hours. If it's between 7.0 and 8.0, you're set. If it's still below 7.0, add more FloMag at the next refill. If it's above 8.0, use less FloMag next time.

You don't need to empty the tank and start over. The new blended media mixes with the existing calcite as water flows through the bed.

One thing to consider: if your system has been running for several years without a full media change, the old calcite at the bottom may have compacted into a clay-like layer. This happens gradually from sediment in the well water settling to the bottom. If you're noticing reduced water pressure or flow, it's time to dump the tank entirely, rinse it out, and re-bed with fresh media. See our servicing your acid neutralizer guide for the full process.

Common Mistakes to Avoid

1. Using too much FloMag

This is the number one mistake. More FloMag does not mean better pH correction. It means overcorrection. Your water goes from acidic to alkaline, and alkaline water above pH 8.0 causes scale buildup on fixtures, damages rubber seals and gaskets, and creates a flat or metallic taste. Stick to the ratios in the table above and test your output.

2. Using FloMag by itself

Never load a tank with 100% FloMag. The reaction is too fast and too aggressive. There's no self-limiting mechanism like calcite has (calcite stops dissolving once the water reaches a certain pH). Pure FloMag will push your pH to 9.0 or higher.

3. Not mixing before adding to the tank

If you pour the calcite in first and then dump FloMag on top, you'll have a concentrated FloMag layer that overcorrects the first water to contact it. Always mix the two media together in a bin before pouring into the tank.

4. Not testing after making changes

Whenever you add FloMag for the first time or change the ratio during a refill, test your output water pH after 24 hours of normal use. A digital pH meter from Amazon costs about $15 and gives you a reliable reading. Test strips from hardware stores can be off by a full pH point, so they're not accurate enough for dialing in a media blend.

5. Buying pre-mixed bags at the wrong ratio

Some plumbing supply houses sell pre-mixed calcite/corosex bags. The problem is you don't control the ratio, and the price is typically double what you'd pay mixing it yourself. Buy calcite and FloMag separately, mix to the correct ratio for your pH, and store the remaining FloMag for years of future refills.

Troubleshooting After Adding FloMag

Output pH is too high (above 8.0)

This usually means too much FloMag in the blend. Solutions:

• Run water through the system for several hours. Fresh water dilutes the initial high reading as the most reactive FloMag granules dissolve first.
• If the pH is still high after a day of normal use, add a bag of pure calcite (no FloMag) on top. This dilutes the FloMag concentration in the bed.
• At the next full refill, reduce the FloMag amount per bag of calcite.

Output pH is still too low (below 7.0)

Possible causes:

• Media level is low. Open the fill port and check. If you can see only a few inches of media, the water isn't spending enough contact time in the bed. Refill with a blended mix.
• Not enough FloMag in the blend. Increase by 1 to 2 lbs per bag at the next refill.
• Tank is undersized. If you have a 1.0 or 1.5 cubic foot tank with a pH below 5.5 and a household of 3+ people, the water may pass through too quickly for the media to react. You may need to size up to a larger tank.
• Old compacted media at the bottom. After several years, sediment and calcite fines settle into a clay-like layer at the bottom that reduces contact efficiency. A full dump and re-bed fixes this.

Water is cloudy or white after adding media

Normal. Both calcite and FloMag produce dust. Run a cold water faucet for 10 to 15 minutes after adding new media. If you have a backwashing system, initiate a manual backwash cycle. The water should run clear within an hour of normal use.

Pressure drop after adding media

If you filled the tank too high (media less than 6 inches from the top), the bed doesn't have room to expand during use. Remove some media through the fill port. For a 2.5 cubic foot tank (13x54), the media level should be about 10 to 12 inches below the fill port opening.

Refilling a FloMag Blend System

Acid neutralizer media dissolves over time. That's how it works: the media sacrifices itself to raise your pH. You'll need to add more periodically.

How often to refill

• Calcite-only systems: Every 6 to 12 months for average usage
• Calcite + FloMag blend systems: Every 4 to 8 months. FloMag dissolves faster than calcite, so blended systems need slightly more frequent refills.

Check the media level every 3 to 4 months by opening the fill port and looking inside with a flashlight. When the level drops 4 to 6 inches from where it was when full, it's time to add more.

How much to add

Typically one to two bags of blended media per refill for a 2.5 cubic foot tank. The goal is to bring the level back up to about 10 to 12 inches below the fill port.

Maintain the same ratio

When refilling, mix the same ratio you used initially. If your system runs on a 90/10 blend, keep adding 90/10. Changing the ratio mid-use won't cause problems, but consistency gives you the most predictable output pH.

For the full maintenance walkthrough (including how to do a complete dump and re-bed), see our servicing your acid neutralizer guide.

Cost Breakdown

The FloMag itself isn't a major ongoing cost because you use so little of it per refill. The bulk of the expense is calcite, which dissolves regardless of whether you blend in FloMag or not. Think of the FloMag bag as a one-time purchase that supplements your calcite for years.

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