Although calcium carbonate reactors have been around for a while, I first saw calcium carbonate reactors about two years ago, when Jeff Turner of Exotic Aquaria showed me videotape of some German reef tanks. Several of them were using these devices to maintain the carbonate hardness levels in their predominantly small polyp stony (SPS) corals. I was very intrigued by the idea of using a reactor, since I was fighting a loosing battle with trying to maintain the Ca and alkalinity levels in my 180G SPS coral tank through the use of Kalkwasser only. Not only did this device have the potential to address my problem; it seemed like a dream come true for a consummate lazy aquarist like me. No adding and mixing Kalkwasser daily, no hassles with trying to increase evaporation and adding humidity to basement, and other hassles that I am sure you are all aware of.

Before delving into the reactors let us first take a look at how it works. The basic idea of the reactor is the reverse process of calcification. A calcium carbonate media is dissolved using carbonic acid (generated by addition of CO2 to water) to provide the Calcium and bicarbonate ions, in the same proportion that is used during calcification.

CaCO₃ + H₂O + CO₂ <-> Ca²⁺ + 2HCO₃^-

So the calcium carbonate reactor is basically a device that brings the three ingredients together in a manner so as to allow efficient production of Ca and bicarbonate.

There are two basic types of designs that have been used to construct the reactor. The open circulation kind and the recirculating kind. The current trend is to use the recirculating type of design, so I will not discuss the open circulation kind. In a recirculating type of reactor {also called the Lobecke Reactor, and described by Hebbinghaus, 1994] the mixture of CO2 and H2O is continuously circulated in the chamber through the calcareous medium in a closed circulation loop. Water from the tank/sump enters the reactor through an inlet and exists through a outlet, both of which are connected to the main recirculating loop [see figure]. Typical recirculation rates a re in several hundred gph, while typical input/output rates are in few liters/hr. Water entering the reactor comes in at tank alkalinity levels and effluent leaving the reactor has much higher alkalinity (depending on the settings). The amount of CO2 added is typically measured in bubbles/min (unfortunately there is no standard bubble size, so this is a dubious measure when comparing different reactors).

All the commercially available reactors basically follow the dame 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. The Knop reactor for example, has a top to bottom circulation loop, whereas the MKR reactor has a reverse flow circulation loop through the media. Some require an external pump to input the water into the reactor, whereas others use gravity or siphon to introduce water into the reactor. You can easily see the variations in design by looking at the commercially available products, but they all essentially do the same thing - provide a mixing chamber for the three ingredients - calcareous medium, CO2 and water. So let us stick to the basic principles and not get into a product review, at this point.

There are basically two adjustments that can be made to the reactors - the amount of CO₂ being added and the effluent flow rate. Keeping the same amount of CO₂ and increasing the effluent flow rate will result in a reduction in the alkalinity of the effluent, Increasing the amount of CO₂ while maintaining the same effluent flow rate will result in a increase in the effluent alkalinity. So, using these parameters we can adjust the resulting alkalinity of the tank.

The most important ingredient in the Ca reactor is the calcareous medium used. It is the only ingredient whose "quality" is attributed to the problems one could encounter when using a reactor. The factors of concern are

1. the chemical composition of the medium

Since the calcareous medium is dissolved in the reactor, it is reasonable to expect that several of the compounds contained in the medium are susceptible to dissolution and subsequent introduction into the tank. Magnesium, Strontium, Phosphates, and other heavy metal salts present in the medium will also dissolve and enter the tank via the effluent. This has both positive and negative effects. On the positive side, one may be able to stop adding additional Strontium and other trace elements. On the negative side we may be adding large amounts of undesirables such as phosphate, copper, heavy metals, etc that will contribute to the problems that I will discuss later. I wish the companies selling these would provide a complete chemical assay of their medium (To be fair to the companies - I have not yet tried calling them to get it either).

 

2. the particle size of the media.

The particle size of the media impacts the total surface area of the calcareous media exposed to the carbonic acid formed in the reactor, and the inter particle spacing in the media. The larger the particle size the less total surface area available for the carbonic acid to react with the medium. So a medium with a larger particle size will dissolves more slowly, and has larger inter particle space which results in less trapping of powdered particles in the medium, and has easier flow through the medium.

 

Smaller particles provide a much large surface area, and will dissolve at a faster rate. This has its own disadvantages. The medium turns to powder (mud) much faster than the larger grained particles. This powder mud being light in weight is easily suspended in the water in the reactor and results in chalky effluent being introduced into the tank. Further, the powder particles tend to stay trapped in the medium and sometimes when the reactor "burps" it spews a lot of chalky effluent into the tank. Using finer grained particles also puts more back pressure on the pump used for circulation.

 

The circulation rate in the reactor is very high compared to the rate at which the effluent is drawn. Hence, I don’t think that the rate of dissolution due to the particle size (with in the size range of most commercially available media) is a sensitive parameter in the operation of the reactor.

 

3. Choice of Media

There are several commercially available calcareous media that can be used in a reactor. Most common ones are the "aragonite" based mediums that are used as "sand" in the aquariums. There are other mediums currently being sold that claim to be designed to reduce the amount of unwanted compounds in the medium. Other medium used range from crushed coral, halimeda chips, oyster and clamshells, etc. The calcite based materials (clam and oyster shells) tend to dissolve more slowly given the lower solubility of calcite as compared to aragonite.

1. Depressed tank pH

For an aquarium at equilibrium conditions there is a certain quantity of CO₂ that can be maintained without reducing the pH. As this amount of CO₂ is increased, additional CO₂ will decrease the pH. Thus when adding the effluent back to the tank, there will always be some additional CO₂ that will be added to the tank, that can result in a drop in the tank pH. This drop in pH will be higher if the tank initially has low alkalinity.

 

In my tank I have not seen any depression in tank pH. In fact, the tank runs at a slightly higher pH since I added the reactor. This is possibly due to the fact that the tank is now running at higher alkalinity values [10-12 dkH] and hence less susceptible to pH drop. I also think that since I am running a downdraft skimmer it may help to "blow off" the excess CO₂.

 

To address the problem of adding back excess CO₂ to the tank, I have experimented with a CO₂ blow out chamber, where the excess CO₂ from the effluent is "blown out" by vigorously aeration using a air pump. Using this I was able to increase the pH of the effluent from around 6.8 to 7.9, before introducing it into the tank. Unfortunately this was not without its own problems. I found that there was quite a bit of precipitation of CaCO₃ in this chamber and have since discontinued its use.

 

 

2. Increase in Phosphates

A common criticism leveled against these reactors is that they can increase the phosphate levels in the tank. The reasons cited are:

1. the medium used contains phosphates which redissolve and are introduced into the tank

 

I have not done any chemical assays of calcareous mediums available to determine which one is high in phosphates. In an Aquarium Frontiers (May/June 1997) article Greg Schiemer stated that the effluent using Caribsea Sea Flor tested at 0.25-.30 ppm of phosphate, but there was no measurable increase in the phosphate levels in his tank. I have seen similar results in my tank. At the rate at which this extra phosphate is being added to the water, it is either being removed by the skimmer or being bio assimilated. If the phosphate issue is a concern to you and is in fact increasing the phosphate levels in your tank, then you may want to try some of the "purer" brands of calcareous medium that are being advertised. I have not used them so I cant really say much about them or their claims. More recently Aquarium Frontiers On-line has an article comparing two brands of calcareous media.

 

2. Since most people using these reactors stop using Kalkwasser, they give up the capability of using the phosphate precipitating properties of Kalkwasser.

 

This may be of some concern in tank where the phosphate transport/removal mechanisms are poor to begin with. Using a "better quality" substrate can minimize the impact of this.

 

3. The drop in the pH of the tank, causes a release of phosphates already bound in the tank substrates (live sand, etc).

I think this is essentially a non-issue. If your tank pH is dropping so much so as to cause the substrate in your tank to redissolve and add back significant amounts of phosphate, then you should be in big trouble already.

(3) Increase in Algae and Diatoms

The increase in available CO₂ in the tank often acts like a fertilizer for the and coupled with the phosphate additions from the medium can fuel an increase in algae growth. Further, the calcareous medium may also contain silicates that are released into the tank, thus increasing the diatom growth.

Whether or not these problems will manifest themselves in an aquarium depends to a large extent on the prevailing conditions of the tank - its ability to degass (or use) excess CO₂, transport/removal of phosphate, availability of herbivores to keep the algae in check, etc. My advice to people thinking of using Calcium Carbonate reactors is to first get your tank running in peak condition without the reactor. Once you are over the typical tank problems such as red slime, hair algae, diatom blooms, and you have a trouble free tank then evaluate whether you really need or want a Ca reactor. Most tanks can be run very well without them, and the only reason to use one on these tanks is the lower maintenance factor.

Once you have decided to add a reactor, how do you go about adjusting it, i.e. determining the effluent flow rate and the amount of CO₂ to be added.

There are basically two adjustments that can be made to an operational reactor - the effluent flow rate and amount of CO₂ added. There is wide range of values at which the reactors are operated by hobbyists. The table below shows a small sample of operating parameter data I have gathered from various aquarists across the country. These tanks range from SPS tanks to tank with only soft corals.

As you can see, there is a wide range of operating parameters that will typically satisfy the alkalinity and calcium needs of different aquariums.

The goal of the reactor is to provide HCO₃- and Ca₂+ at the rate at which they are being consumed in the tank. So we need to find out how much is being consumed and how much is being generated by the reactor and balance the two values.

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 medium, and hence a higher alkalinity in the effluent. This increase in alkalinity of the effluent will be seen upto a pH range of 6.3-6.5 and any further decrease of the pH in the reactor to increase the solubility of the substrate 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.

Now let us look at how much alkalinty is being generated by a reactor and adjustment to meet the tank requirements. To initially set the reactor follow the manufacturer's recommendations. (The following discussion is based on several personal conversations with Dr. Craig Bingman, Rod Andrews and the experience of several others in the Fishroom at kplace.monrou.com).

Let us assume that the reef system contains T liters of water, and the effluent flow rate is L liters/hr. 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. Let us say this is d meq/L.

Assuming no calcification and use of alkalinity, this will result in a an increase in tank alkalinity that is given by the following formula:

Increase in tank alk/day due to the reactor = (d x L x 24)/T - (1)

Now measure the tank alkalinity after a day. The difference between the increase in alkalinity due to the reactor and the actual increase in alkalinity will give you the daily consumption of alkalinity for your tank. Let us say this value is c meq/L.

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 3 ways in this can be achieved.

1. Only adjusting d - the increase in effluent alkalinity
2. Only adjusting L - effluent flow rate
3. 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 inject too much CO₂ so as to cause the pH in the reactor to drop below 6.3, I increase 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 users needs.

I don’t want to bore you with the math and chemistry details but I feel that a basic understanding is necessary to avoid the trial and error, and test and adjust solutions. You can use the above equation to calculate, for example, what the value of d should be, given a certain effluent rate and the desired increase in alkalinity - rather than making wild guesses and adjustments.

Calcium carbonate reactors are a very useful device, and when used with the proper care and understanding of its advantages, and associated problems addresses a constant struggle faced by most aquarists - balancing Ca and alkalinity needs and providing them at the rate at which they are consumed. For a consummate lazy aquarist like me it has been worth every penny. I'd like to end with a word of caution. Calcium carbonate reactors are not a magic device that will make your problems go away. In fact I have seen several instances where this was the attitude, and the reactor was used on tanks that already had existing problems of algae growth, poor phosphate transport, poor water quality, and all the reactor did was to aggravate these problems. The increase in available CO2 fueled the algae growth and the possible additions of phosphate from the media increased the cynobacterial growth. My advice to people planning to use a calcium carbonate reactor is to first get your tank running in top shape without a reactor and then think about adding one.

I built myself a DIY one, and I know I will be deluged with requests for plans. I have a sketch and notes for building one here .