How to Use FloMag (Corosex) in Your Acid Neutralizer

Aidan "People ask me, can I use FloMag by itself? And I'm like, no. If you used plain FloMag and filled up the whole tank, it would turn into one big brick and the tank would be useless."

That is the first thing I say about FloMag, because it is the one mistake that costs people a tank. The rest is simple: a few pounds in each bag of calcite, mixed dry in a storage bin, and your pH comes up where calcite alone could not get it. One buyer wrote last spring that a one-page mixing instruction would have saved him a lot of headaches. This is that page.

What Is FloMag (Corosex)?

FloMag is a granular magnesium oxide media used inside acid neutralizer tanks to raise the pH of acidic well water. Calcite, the white limestone that fills the tank, raises pH slowly and stops on its own near neutral. FloMag has four to five times the pH-raising power of calcite and dissolves a little quicker, so a few pounds of it in each bag of calcite pull a low pH up where calcite alone would stall.

FloMag and Corosex are the same thing under two names (Corosex is Clack's name for it). A New Jersey caller asked me in August what the difference was and I told him what I tell everyone: same product, and they call it FloMag because FloMag sounds better. It is a natural mineral, certified by NSF for drinking water, not a chemical feed. The one line on the manufacturer's sheet that matters at home: keep it dry, or it cakes.

The bag is white with blue printing. A New York buyer called me this month with five bags of calcite on his garage floor and no way to tell whether one of them was FloMag; it was not, because his order never had it. If every bag you have is blue, you have calcite.

Do you need FloMag?

pH 6.0 to 6.9 (mildly acidic): Calcite only, in any tank size. This covers the majority of residential well water. Of the acidic wells my customers have had lab-tested, about four in five came back at 6.0 or higher, so most people reading this do not need the bag. A Virginia caller pricing the 2.5 cubic foot tank in July asked me about the FloMag kit on the product page and I told him to skip it; his pH was above 5.5 and calcite alone would carry it.

Below 6.0 it depends on the tank, because the bigger tank holds more calcite for the water to pass through.

The order form on the 2.5 cubic foot non-backwashing neutralizer asks whether your pH is under 5.5 and adds the bag when you say yes ($1,695 with FloMag, $1,495 without). The 1.5 asks the same at 6.0 ($1,495 with, $1,295 without). If you already own a tank, the bag by itself is $225.

How many pounds of FloMag per bag

Two to three pounds of FloMag per 50-lb bag of calcite is the standard mix, and the lower your pH the more goes in, up to 5 pounds a bag between pH 4.0 and 5.0. There is no single number because the tank size and the pH both move it. These are the numbers I actually give on the phone:

The goal is water that comes out between 7 and 8 while you are using it. Get the number for your pH and your tank before you mix, and then mix that, not more.

Ask Aidan

Text me your pH for the exact mix

Send your pH and your tank size. I'll text back the pounds per bag for your water; it takes me about a minute.

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

8 AM to 5 PM Eastern, seven days a week. A real person, usually Aidan.

How to mix FloMag with calcite

Mix one bag at a time, dry, in a storage bin, and put each bag's mix in the tank before you start the next one. You need a big plastic storage bin (the gray ones at Walmart are about twelve dollars), a food scale or a big measuring cup, and your two hands. Do not rinse or wash the calcite first. Wet calcite will not go through the funnel; it would take you four hours to load the tank. Everything goes in dry.

1. Dump one bag of calcite into the bin

   The whole 50-lb bag, into an empty bin. One bag at a time is the point: each bag gets its own measured FloMag, so the tank ends up with the same mix top to bottom. That even mix is what keeps the pH steady while you are running water, and it is why a blended tank does not need service any more often than plain calcite.

2. Weigh out the FloMag for your pH

   Two pounds at 5.5 on a 2.5 cubic foot tank, more as the pH drops, per the table above or the number I gave you on the phone. A food scale is easiest. No scale: use a big measuring cup, and remember it is 16 ounces to a pound. Measure it. Do not eyeball it.

3. Mix it by hand until it looks even

   Pour the weighed FloMag over the calcite and turn it over with your hands (gloves are a good idea; it is dusty). Both are granular, so they mix easily. FloMag is a little darker and coarser than calcite, and you can see when it is spread evenly. If you can still see a pocket of FloMag, keep going.

4. Pour the mix through the funnel

   The funnel that came with the neutralizer goes in the fill port on top of the tank. Scoop or pour the bin's contents in, dry, taking your time so it does not bridge in the funnel. Then go back to step one with the next bag.

   

5. Repeat for every bag you have

   A 2.5 cubic foot tank takes five bags of calcite, so you do this five times. Roll the FloMag bag shut and keep it somewhere dry. It does not go bad; it just cakes if it gets damp. You will use it again at your refills for years.

   

6. Run the water until it clears

   Fresh media makes the water milky for a while. Run a cold faucet until it clears. Then use the house normally for a day and test the pH at a tap. Between 7 and 8 while water is running means the mix is right.

   

A minute and a half on camera: the ratio, the bin, why you never use FloMag alone, and how to add it to a tank that already has calcite.

Adding FloMag to a tank with calcite

You still mix the FloMag with fresh calcite in the bin; the only question is how much room the tank has. If the tank is about half empty, make one 50-lb bag's worth of mix and add it on top through the fill port. The new blend works with the calcite already there as water moves through. If the tank is still nearly full, there is nowhere to put it: take the control valve off, put a wet vac down the tank and pull out enough calcite to make room for a bag of the mix. That is how we did it as a service company and it is what I tell customers to do now.

What you cannot do is scoop a little of one bag and a little of the other into the port and hope it mixes on the way down. It does not. That is the Delaware owner's story in the next section.

One more thing to check before you buy the bag: the tank itself. A Georgia caller in January had a 10-inch gravel-bottom tank, raw pH about 5, and water coming out at 6.6 on plain calcite. He wanted to know if a bag of FloMag would finish the job. In a tank that small at that pH it would take a lot of FloMag, and the honest answer was that he had the wrong tank; at a pH of 5 we use the 2.5 cubic foot tank.

Three ways FloMag goes wrong

Too much FloMag

Years ago I sold a system to a contractor and told him two to three pounds of FloMag per bag of calcite. He dumped in the whole 50-lb bag. The pH shot up to 11, and he had to empty the tank, buy all new media and start over. A buyer in 2020 did a smaller version of the same thing: he filled the tank three-quarters full with a 10 percent FloMag mix, five pounds a bag, and his pH went from 5.4 to over 10. He shop-vacuumed the media down to half and it settled in the high 8s. If your output pH is over 8, you have too much FloMag in the tank. Pull some media out and go lighter on the FloMag at the next refill. When I give you the number for your water, mix that number, not more.

FloMag poured in unmixed

A Delaware owner opened her tank for a refill in August and found a block of media twelve to fifteen inches thick, very hard, six or eight inches below the top of the tank. She could not lift the bags, so for years she had put in a couple of scoops of calcite, then a little FloMag, then more calcite, straight into the port. FloMag that is not mixed through the calcite sets up together. It turns into something like concrete. If you find a block like hers, break it up with a broomstick or a length of pipe and get the chunks out; do not leave them in there. Then refill with a mix made in the bin.

The tank is too small for the pH

FloMag makes up for a pH that calcite alone cannot reach. It does not make up for a tank that is too small to hold the calcite in the first place. The Georgia caller above is the example: a 10-inch tank at a pH of 5 will not hold neutral no matter how the bag is mixed. If your tank is a 1.5 or smaller and your pH is under 5.5, the answer is the 2.5, with the blend in it.

Does FloMag make the water harder?

A little, but not much more than plain calcite already does. Every acid neutralizer adds hardness, because the calcite dissolving into the water is calcium, and the FloMag dissolving into it is magnesium. Calcite's share is about 3 to 6 grains per gallon (how the calcite dissolves); a few pounds of FloMag per bag adds a little on top of that. If the hardness after the neutralizer bothers you, a softener after the neutralizer takes it back out.

What a bag of FloMag costs

FloMag only comes one way, a 50-lb bag for $225 with free shipping, and at a few pounds a refill one bag covers many years of refills. I tell people the bag will last them seven, eight, nine years. Calcite is $145 a bag and you use a bag or two at every refill, so the calcite is the ongoing cost and the FloMag is closer to a one-time buy. It is more expensive per bag than calcite, but you are not using the bag; you are using a couple of pounds of it, and the rest sits dry on a shelf until the next refill.

What buyers said after mixing it

These are verified purchases, in the buyers' own words, newest first. Two of them used more FloMag than their water needed, which is the other way the mix goes wrong.

Frequently asked questions