Building Your Own Calcium Carbonate Reactor

Before I get into the details of building a calcium carbonate reactor, let us first take a very brief look at the basic principle behind it. The easiest way to view the functioning of the reactor is as a reverse calcification process. The idea is to dissolve a calcareous media in such a manner that it provides both bicarbonates HCO3- (alkalinity) and calcium (Ca++) ions in the same ratio as they are consumed during calcification. Effective dissolution of the calcareous media requires that the solute be at an acidic pH. Because saltwater is usually at a pH of 8.0 and higher, we need some way of reducing the pH in the reactor to aid the dissolution of the media. Carbon dioxide (CO2) is used to help lower the pH in the reactor, and the general reaction form is as follows:

CaCO3 + H2O + CO2 <——> Ca2+ + 2HCO3-

So, the calcium carbonate reactor is basically a device that brings the three ingredients — water, CO2 and calcium carbonate — together in a manner so as to allow efficient production of HCO3- and Ca++ ions. All reactor designs basically follow the same principle, but differ in the techniques to create the circulation loop, the manner in which CO2 is injected, how the water is input to the reactor, and how the effluent is drawn off. We will just stick to the basic principles and look at building your own reactor.


  • Hacksaw
  • Measuring Tape
  • Electric Drill
  • ½-inch American National Standard Taper Pipe Thread (NPT) (tap and drill set) — If gluing the fittings to the reactor body then these are not necessary, but you will need an appropriate-size drill.
  • 1/8-inch NPT (tap and drill set)
  • The most common reactor design that is currently used is the closed recirculating reactor. In a recirculating type of reactor (also called the Löbbecke reactor, described by Hebbinghaus 1994), the mixture of CO2 and H2O is continuously circulated in the chamber through the calcareous media in a closed circulation loop. Water from the tank/sump enters the reactor through an inlet and exits through an outlet, both of which are connected to the main recirculation loop. Typical recirculation rates are in several hundred gallons per hour, while typical input/output rates are in few liters per hour (L/hr). Water entering the reactor comes in at tank alkalinity levels and effluent leaving the reactor has a much higher alkalinity (depending on the settings).


    Only the parts for the base reactor are listed below. Additional parts required for modifications are listed where and as needed. Living in a small town, I cannot usually find parts in a store, so I have listed some sources from the catalogs I have at hand in case you also have difficulty obtaining these parts at the local hardware store. I am also listing some approximate prices so you have an idea of what costs you are getting into.

    Materials List for Mixing Chamber Assembly
    1 4½-inch OD 1/8-inch thick
    24-inch long acrylic tube
    Eco-Systems (AE)
    TA5 1 $20
    2 4-inch PVC closet flange
    with test cap
    Local Hardware Store (LH) 1 $5
    3 4-inch PVC female adapter
    (FNPT* x Slip)
    ALSCO Industrial
    Products (ALS)
    1813-040 1 $4.13
    4 4-inch PVC cleanout plug
    with male pipe thread
    ALS 1812-040 1 $2.22
    5 ½-inch PVC male adapter
    (MNPT** x Slip)
    AE 436-005 2 33¢
    6 ½-inch PVC pipe "legs"
    (cut from PVC pipe)
    LH 3
    7 Egg crate —
    5 inches x 5 inches
    LH 1
    8 Fiberglass window screen mesh
    (1 square foot)
    LH 1

    Materials List for Re-Circulation Loop
    and Input/Output Connections
    9 ½-inch PVC union
    (Slip x Slip)
    AE 457-005 3 $1.55
    10 ½-inch PVC elbow
    (Slip x Slip)
    AE 406-005 3 36¢
    11 ½-inch PVC Tee
    (Slip x Slip x FNPT)
    AE 402-005 2 58¢
    12 Tubing adapter —
    elbow ½-inch MNPT x ¼-inch barb
    AE 62085 1 62¢
    13 ¼-inch needle valve for water ALS 4003-002-A 1 $32.80
    14 Tubing adapter —
    1/8-inch NPT x ¼-inch (or 3/16-inch) barb
    AE 62001 1 24¢
    15 Supreme Mag2 pump AE MD2 1 $37.90
    16 ½-inch PVC 90 street ell
    (MNPT x Slip)
    ALS 410-005 1 48¢
    17 ½-inch PVC female adapter
    (Slip x FNPT)
    AE 435-005 1 38¢
    18 Nipple ½-inch MNPT x ¼-inch MNPT AE 62083 1 51¢
    19 ½-inch MNPT x ½-inch barb AE 62018 1 46¢
    20 ½-inch PVC pipe LH 3 feet
    * = Female National Pipe Thread
    ** = Male National Pipe Thread

    Item Source Catalog
    ½ Pint PVC and CPVC Clear Primer ALS P-6866C-H $3.20
    ½ Pine PVC Solvent Cement ALS 700-H $3.85
    1 Pint WELD-ON 1802 AIN $5.60
    Teflon Tape — ½ inch x 260 ALS S260 79¢
    Marine Goop or Plumber’s Goop LH $5.50

    Company Phone Number Web Address
    Aquatic Eco-Systems (800) 422-3939 http://www.aquatic-eco.com/
    ALSCO Industrial Products, Inc. (800) 849-3030 http://www.alscoind.com/
    US Plastics Corp. (800) 537-9724
    AIN Plastics, Inc. (800) 431-2451

    Purchasing and assembly notes

    Most of the PVC parts can be easily bought at a home supply or hardware store. It may not be easy to find a 4-inch PVC closet flange with test cap intact — most of the ones I have seen in stores lately are open flanges with the knockout test cap portion removed. Try to find one with the knockout intact. If not, you can get a plastic test end cap and glue it on. Sometimes you can find 3- x 4-inch PVC closet flanges more easily than the 4-inch ones. These can also be used, but will require some additional pieces to accommodate the 4½-inch acrylic tube (described in the section on assembly).

    For the body of the reactor, you can use either 4-inch PVC pipe, 4-inch clear PVC or the 4½-inch OD acrylic tube. I prefer to use the 4½-inch OD acrylic tube because it works well with 4-inch PVC fittings, and if you like to see into the reactor, then it is economically a better choice to use than the 4-inch clear PVC, which tends to be more expensive. Unfortunately, it is not easy to get a 2-foot piece of the acrylic tube via mail order, as most places will only sell it in 5-foot increments, or have a minimum order that will not be met with the purchase of just this piece. The best source I have found for precut pieces of acrylic tubing in sizes that work with PVC fittings is Aquatic Eco-Systems, Inc.

    To assemble the threaded fittings to the reactor body I prefer to cut pipe taps rather than just gluing the fitting into a hole. Drilling the appropriate-size hole and gluing the fittings is perfectly fine if you don’t want to make the investment in pipe taps. For those of you interested in making National Pipe Tap (NPT) threads, the following information may be of interest. The ½-inch NPT can be made by first drilling a 23/32-inch hole and then using a ½-inch NPT tap. Attaching the fitting with the threads will allow for easy disassembly of the recirculation loop (in case you need to use the chamber for other purposes or redo your design).

    Pipe taps can be purchased from MSC Industrial Supply Company. I find it very handy to have 1/8-inch, ¼-inch and ½-inch NPT taps, and corresponding drills. The table below provides the information of the drill, taps, and part numbers from the MSC Industrial Supply Catalog.

    Pipe Tap
    Part Number
    Tap Drill
    Part Number
    1/8 inch 04350021 $3.17 11/32 inches 01180223 $1.27
    ¼ inch 04350039 $3.58 7/16 inches 01180280 $1.97
    ½ inch 04350054 $6.73 23/32 inches 01340462 $7.99
    ¾ inch 04350070 $10.67 15/16 inches 01340603 $11.10

    To glue the acrylic to PVC ideally you will need an adhesive that is designed to glue acrylic to PVC (e.g., Weld On #1802 (or Weld ON #40). These can be bought from AIN Plastics. Although, I have used regular general-purpose PVC cement and it has held well so far.

    For those of you who would like to know more about working with plastic threaded fittings and making leak-proof fittings, Lasco Fluid Distribution products provides excellent information, including “How to Avoid Problems with Threaded Plastic Fittings”. To learn about working with plastic solvent cement you can read “Solvent Cementing Procedure” — it’s in a nice enjoyable cartoon format. Another good article on the same site is “Leak-free PVC Joints”. A tube of Marine Goop will come in very handy in sealing the joints from the inside of the reactor to prevent any leaks caused by sloppy gluing.

    Figure 1

    Reactor construction details

    For the sake of simplicity, I will divide the basic reactor into four parts: 1) the mixing chamber, 2) the recirculation loop, 3) the input/ouput from the tank and 4) the CO2 injection system. Figure 1 shows a sketch and a picture of the reactor.

    1) Mixing chamber

    This is the main body of the reactor and is essentially a closed container that will house the media and the CO2-enriched tank water. The basic construction of this chamber is shown in Figure 2.

    Assembly Steps (Refer to the “Materials Lists” above for the parts numbers listed here in parentheses)

    Step 1 — Building the mixing chamber.

  • To build the mixing chamber, glue the 4-inch PVC closet flange (2) to one end of the acrylic tube (1) and the 4-inch female adapter (3) to the other end. If you are using the 3- x 4-inch PVC-DWV closet flange, first cut a small length of 4-inch PVC pipe that will fit over the outer diameter of the closet flange (see Figure 3). Then use a 4-inch PVC female adapter (Slip x Slip) to fit over the 4-inch PVC, and then the 4½-inch OD acrylic tube will fit inside the coupling. My current reactor uses this construction.

    Figure 2
    Step 2 — Attachment for the recirculation loop.

  • Attach a ½-inch PVC male adapter (5) to the square end of the 4-inch threaded cap (4). You can either attach this by drilling a hole and gluing the adapter in or by making a ½-inch NPT tap into the cap.

  • Attach a ½-inch PVC male adapter (5) to the bottom of the reactor body, about 1½ inch from the bottom. Use the same method to attach the adapter as in the first part of Step 2. However, you need to be slightly more careful because you may now be drilling into the acrylic. Acrylic has a tendency to grab onto the drill bit and stick. Don’t stop the drill while the bit is in the acrylic. Pull the drill out of the hole before stopping the drill motor. When the drill bit is jammed in the acrylic, it is quite easy to crack it while trying to remove the jammed drill bit.

    Step 3 — Support for the media.

  • Because this is a reverse-flow recirculating reactor, the water flow is going to be from the bottom of the reactor to the top. We need to raise the media above the water input (the PVC male adapter at the bottom of the mixing chamber). This will provide for a more even flow through the media. The support is constructed from a plastic egg crate, placed on three legs, and a screening material that is placed over the egg crate to prevent the calcareous media from falling through.

  • Cut the egg crate into a circle to slide into the inside of the mixing chamber.

  • Cut three lengths of PVC pipe tall enough to rise above the PVC adapter at the bottom. Glue these “legs” to the bottom of the flange, inside the reactor body. These three legs will provide the support for the egg crate circle to sit on.

  • To make the screen to prevent the calcareous media from falling through, I used a fiberglass window screening material. Double up the window screening material and cut a circle about ½ to ¾ inch larger than the inside diameter of the acrylic tube.

  • Place the screen on the egg crate and goop (glue) the edges of the circle (it will fold over because the circle is bigger than the tube) to the acrylic tube.

  • To prevent the calcareous media from entering into the pump and the recirculation plumbing, again cut a doubled-up piece of window screening material and goop it to the inside of the 4-inch threaded cap used to close the mixing chamber.

    Figure 3
    Modification Notes — As an alternative to using the window screening material, you can use large open-celled foam that will allow water to pass freely. Some people prefer to use the open-celled foam, as it also tends to trap the fine particles of media and prevent them from going back into the tank and clouding the tank. I don’t particularly like it because, over time, this can get clogged and lead to reduced flow in the recirculation loop.

    2) Recirculation Loop

    This is the part of the construction that will take the water in the mixing chamber and keep it recirculating through the medium. The basic recirculation loop used here is a reverse flow through the media. I like this for several reasons — it keeps the media from compacting and, because the particles tend to stay loose, it prevents channeling. The downside is that the intake to the pump can run dry if the water level drops inside the reactor to the point where it cannot be pushed through the loop. You can easily modify the design to run in a top-down flow scheme, if this becomes a big concern, simply by changing the way the pump is mounted.

    The basic construction here is very easy and similar to any plumbing you may have done in setting up your tank and is shown in Figure4. The unions will help in assembly of the reactor and in taking it apart for cleaning the pump and when adding media. I have raised the pump so that the complete assembly takes up a smaller footprint.

    Figure 4
    Modification Note — You will notice in the picture of the version I am currently using that I have replaced a piece of PVC pipe with a flex hose, clamped to the pipe. I did this because, over time, the end cap on top of the reactor started sitting deeper and deeper into the threaded female adapter and I had to repeatedly cut the pipe to shorten it to avoid the difficulty in re-assembly.

    3) Input and Output from Tank

    The water enters the recirculating loop via a needle valve (13). The needle valve is needed because most reactors are only operating at 20 to 50 milliliters per minute and this adjustment is difficult to make with a ball or gate valve. A small powerhead is used to pump water from the sump into the reactor through the needle valve.

    The output (12) is a tubing adapter — elbow ½-inch NPT x ¼-inch hose barb — fitting placed at the highest point of the recirculating loop. The output basically is the overflow from the reactor. The amount of output will depend on the rate of input. I initially had the needle valve at the output, but it tended to clog easily due the calcium deposits formed by the effluent running through the needle valve. The reason for restricting the barb fitting to a smaller size was to make it difficult for the CO2 to escape from the reactor.

    Important Note — When attaching the powerhead to the input port via tubing, make sure you use a check valve to prevent water from the reactor draining back into the sump when the reactor is shut down for maintenance. Also, this check valve will prevent water from the reactor draining out in case of a power failure or failure of the powerhead. If the powerhead fails and water drains out, it will cause the re-circulation pump to run dry and burn out. The Polypropylene Check Valve (Part # 228215 from Aquatic Eco -Systems; $2.95) works well, as it is designed to work with different-size tubing and does not have to be in a fixed orientation to work.

    4) CO2 Injection System

    CO2is introduced into the reactor through a 1/8-inch NPT x 3/16-inch hose barb fitting (14). The fitting is attached to the 90-degree elbow by drilling and tapping a 1/8-inch NPT into the elbow. Once again, this can also be glued in place instead. The CO2 input is placed so that the CO2 bubble can easily enter the pump and be broken up by the impeller into smaller pieces to facilitate mixing of the CO2 and the water.

    Figure 5
    The complete CO2 injection system is shown in Figure 5 and consists of the following items:

  • CO2 tank — The CO2 tank can be purchased for a reasonable price from a welding supply store, brewery supply, or fire safety supply store. The tanks come in several sizes: 2½, 5, 10, 20 pounds. and even larger. I am currently using a 20-pound CO2 tank and it lasts me for over six months between refills. The cost of refilling a 10-pound tank versus a 20-pound tank is not much different (only a $2 to $3 difference), so it may, in fact, be economical to get the bigger tank.

  • Regulator — Regulators are used to reduce the high pressure in the CO2 tanks (around 800 pounds per square inch [psi] or so, depending on the temperature) to more manageable and safer pressures, typically 0 to 60 psi. Most regulators are adjustable and can be easily adjusted using a knob (or screw) provided on the regulator.

    Regulators come in either a single gauge or dual gauge model. In the single gauge model, the gauge provides the reading for the output pressure only. The dual gauge regulators provide an additional gauge that shows the pressure inside the tank. This helps in monitoring when the CO2 is running out, although with CO2, pressure remains fairly constant throughout. Once it starts dropping it drops very rapidly, so the gauge for monitoring CO2 tank pressure can be eliminated. Regulators can also be purchased from the same sources as the CO2 tanks. My regulator is currently set at 20 to 25 psi.

  • CO2 Needle Valve — The regulators do not provide fine enough control for adjusting the CO2 injection rate. A needle valve suitable for handling air/CO2 is used to achieve this. I have tried two different valves, the ARO and the Whitey (model number B-ORF2). I was not too happy with the ARO valves and found it a little difficult to adjust for the application.

    The names of the dealers for the ARO valves in your area can be obtained from ARO Corp. (419-636-4242). Grainger also carries these valves: 1) ARO model FO1, NPT size 1/8 inch, Grainger Stock No. 6ZC07 and 2) ARO model F02, NPT size ¼ inch, Grainger Stock No. 6ZC08.

    I am currently using the Whitey Model no. B-ORF2 valve (around $25). I was able to purchase it from the Pittsburgh Valve and Fitting Company (P.O. Box 4155, 49 Meade Ave., Bellevue, Pittsburgh, PA 15202; 412-761-3212 or FAX 412-761-2486).

    Another valve that I have not tried, but that has been recommended, is the slightly more expensive Nupro B-4MG2 valve. This valve can also be obtained from the company listed above. If you have trouble finding these valves contact a Swagelock dealer in your region. Swagelock is the parent company for Nupro and Whitey.

    Figure 6

  • Bubble Counter — Provides a visual means of determining how much CO2 is being introduced into the system. The visual feedback is obtained by introducing the CO2 under water, and counting the bubbles formed by the CO2. The CO2 flow rate in bubbles per minute is not an absolute measure because there is no such thing as a standard bubble size, but it provides a relative reference on how much CO2 is being added. The two basic designs used in the construction of a bubble counter are shown in the Figure 6. I am using “Design - A.”

  • CO2 Check valve — Is used to prevent water from flowing back into the CO2 injection system.
  • Air line Tubing — Is used to make all the connections for the flow of CO2. CO2-safe tubing is typically recommended, but I have had no problems with using standard air line tubing. It could get brittle over time, but is cheap enough that it can be easily replaced.

    Modification Notes — A flow control valve can in fact replace the CO2 needle valve and bubble counter and is cheaper than the two items combined. This valve not only provides the visual feedback, but also provides a reading on how much CO2 is being injected in a more quantitative form (e.g., cubic centimeters per minute; cc/min). The flow control valve I am using is the Dwyer RMA-151-SSV Flowmeter, range 5 to 50 cc/min air with stainless steel valve. I find I am using about 20 cc/min of CO2.

    Reactor operation

    Release the union at the top, remove the 4-inch PVC threaded cap, and fill about three quarters of the reactor mixing chamber with a calcareous media. There are various substrates available on the market (see “Biochemistry in the Aquarium”, Aquarium Frontiers Online, August 1997, for a detailed discussion on these substrates). In case your are curious as to what substrate I am using, it’s Geomarine by Carib Sea. I happen to like the coarse material, as it does not turn into “mud” as easily as the SeaFlor media I was using earlier and it tends to trap less particulate matter in the reactor without compromising, to a large extent, the surface area available for reaction.

    After filling, make sure the threads are clear of any media particles. Using Teflon tape to coat the threads on the male end, tighten the cap, using a large wrench, by grasping the square projection on the cap. Make sure you tighten enough so you don’t get a leak. I personally found it difficult to get a good seal on the 4-inch threads with the Teflon tape and have now switched to using Teflon Pipe Dope, available from ALS in a ½-pint size (part # TF8, $7.38). This is a soft setting compound that seals pretty well and also allows the threads to be taken apart for refilling. It does seem nasty at first, but I have used it without even letting it dry and it has not affected the tank in any visible way.

    To get the reactor started, first open up the needle valve controlling the water input and start up the powerhead to fill the reactor completely with water. Give the reactor a few shakes to drive the entrapped air out of the media. Let the water fill up the complete mixing chamber and the recirculation pipes. Letting the water run for a while, you will get a steady stream of water from the output. Now start up the recirculation pump. You might see that the media lifts and separates into two regions —don’t worry about it, it will not affect the operation of the reactor. As the media is stirred, some more entrapped air will be released. Now cut back the flow of the input water via the needle valve to a steady drip. Turn on the CO2 until you get about 10 to 15 bubbles per minute.

    There are basically two adjustments that can be made to an operational reactor — the effluent flow rate and the amount of CO2 added. In the steady state, the trick is to balance the output of the reactor with the daily consumption of alkalinity in the tank.

    Increasing the CO2, while maintaining the effluent flow rate, results in a decrease in the pH of the reactor and hence an increase in the solubility of the CaCO3 media, and hence a higher alkalinity in the effluent. This increase in alkalinity of the effluent will be seen up to a pH range of 6.3 to 6.5 and any further decrease of the pH in the reactor will start resulting in decreasing alkalinity. Increasing the effluent flow rate, while maintaining a fixed amount of CO2 will result in an increase in pH in the reactor and hence a reduction in the alkalinity of the effluent. By adjusting both the effluent flow rate and the CO2 injection rate, a fixed pH can also be maintained in the effluent. Some advocate maintaining a fixed pH of around 6.5 in the reactor.

    I find it much easier to adjust the reactor based on the alkalinity output rather than the pH measurement. I would recommend first setting up an effluent flow rate so it flows in a continuous steady drip, and then making adjustments to the CO2 flow rate to increase or decrease the alkalinity of the output. For most tanks this approach will work fine, but if you have a heavily loaded small-polyped scleractinian (SPS) coral tank, then it may require you to increase both the effluent flow rate and the CO2 injection rate. A certain amount of fine-tuning is required to adjust the reactor for your particular system.

    Having a good estimate of the daily consumption of the alkalinity in the tank and understanding some of the “reactor math” can help in eliminating some of the trial and error in fine-tuning the reactor. Let us assume that the reef system contains T liters of water, and the effluent flow rate is L liters per hour and the estimated daily alkalinity consumption is c milliequivalents per liter (mEq/L) per day. Now, measure the alkalinity in the tank and the alkalinity of the effluent. The difference between the two values will give you the increase in alkalinity due to the reactor — call this d (mEq/L) — as follows:

    alk/day added due to the reactor = (d x L x 24)/T Equation 1

    So, now we need to adjust the reactor so that the daily increase due to the reactor is approximately c mEq/L. This will give us the setting at which the reactor will replenish the alkalinity that is consumed daily.

    Looking at the Equation 1, we can see that there are three ways this can be achieved:
    Only adjusting d — the increase in effluent alkalinity
    Only adjusting L — effluent flow rate
    Adjusting both d and L.

    The effluent alkalinity can be increased (or decreased) by correspondingly increasing (or decreasing) the amount of CO2 and keeping the effluent flow rate constant. This provides one convenient way of tuning the reactor output to the aquarium needs. When increasing the amount of CO2 added care must be taken to keep the pH level above approximately 6.3. I personally use this approach to adjust my reactor. If I find that I have to injected too much CO2 so as to cause the pH in the reactor to drop below 6.3, I am better off also increasing the effluent flow rate through the reactor.

    Increasing the flow rate will result in a decrease in effluent alkalinity if the CO2 flow rate is not simultaneously increased. Several manufacturers recommend adjusting both the flow rate and the amount of CO2 simultaneously to maintain a constant pH (about 6.5) in the reactor and hence a constant alkalinity output in the effluent. I prefer having to just adjust one parameter — the CO2 flow rate. Both approaches will satisfy the needs of the user, but the key is to balance the daily consumption to the daily addition of alkalinity.


    I have found the reactor to work fairly well, but it has a few minor problems. It is not easy to open and close the reactor because sealing the 4-inch threaded end cap is sometimes not easy. But, this only has to be done once every three to four months. This problem can be eliminated by using a flanged opening, but would require special equipment and a higher skill level to make. Over time, the output from the reactor slows due to calcium buildup and clogging of the output, but this can be easily fixed by adjusting the needle valve. Any reactor you use will require some amount of fine-tuning to balance the alkalinity needs with consumption.

    Overall, the use of a calcium reactor has moved me another step closer toward the ultimate goal of becoming the lazy aquarist who spends time enjoying the tank, rather than working on the maintenance chores — other than trimming my fast-growing SPS corals, of course. If you have any further improvements you would like to suggest or have any comments about the article, please feel free to contact me at sjoshi@psu.edu

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