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Q10910 - INFO: Reactors and the Reef Aquarium by Keith MacNeil, a Marine Depot Staff Member
One goal many aquarists share is to make their reef aquarium maintenance as easy as possible. One way to simplify reefkeeping is to include one or more types of reactors in your setup. Reactors reduce the number of chemicals you normally dose manually to maintain water quality and can move you one step closer to aquarium automation.

There are a few different kinds of reactors for reef aquariums. These few types can be further broken down into two categories:
  1. Reactors that Add
    Calcium and kalk reactors add elements to aquarium water
  2. Reactors that Take Away
    Media and denitrate reactors remove elements (like dissolved organics) from aquarium water
In this week's article, we will cover the four most popular types of aquarium reactors. We'll explain how they work, their benefits and what it will take to set them up and run them with your aquarium.


In a reef tank, calcareous organisms like corals and clams along with calcareous algae, such as coralline algae, utilize the calcium dissolved in the water as one of the components to build their skeletons or shells. To keep a constant level in our reef tanks we need to supply calcium into the water column. One of the ways to accomplish this is the use of a calcium or kalkwasser reactor.

Calcium Reactors

Calcium Reactors What You'll Need
The reactor itself, a feed pump (or small powerhead) to pull water from your tank or sump through the reactor, a CO2 regulator (preferably with a solenoid), a CO2 tank, a calcium based media and some flexible tubing (for hooking up the CO2 as well as the feed pump). A pH controller is also nice to use with a calcium reactor, but is not a necessity.

The Basics: How Calcium Reactors Work
Water is pumped (or gravity fed) from the main tank or sump into the calcium reactor where it is mixed with carbon dioxide (CO2). The mixing of CO2 with water causes a drop in the pH of the water. The lower pH allows the calcium-based media to dissolve, releasing calcium (and other elements) into the water. This calcium-enriched water (called the effluent) is then fed back into the sump or main tank to replenish calcium that has been utilized.

Reactors have a recirculating pump to help aid in the dissolving of the media within the reactor. This pump helps move water through the reactor more efficiently and allows the water to pass through the media multiple times before it is returned to the tank. This enriches the water with higher levels of calcium than if it were to flow only through the reactor once in a single passing. Some reactors are set up to push the water through the media from the top down (down flow), while others push the water up through the media (up flow or fluidized). I wouldn't get too hung up on deciding which would be “best” for your system; I have found both types work extremely well.

A Few Additional Notes on Calcium Reactors
  1. Calcium reactors are best for maintaining the calcium levels in your aquarium. If the calcium levels in your aquarium are already low, you should assist your reactor by using a calcium supplement to raise the level and let the reactor maintain it from there.
  2. The calcium needs in your tank will change over time. As you add more corals and the corals themselves grow, you'll need to occasionally adjust your reactor. There are actually two adjustments you can make. The first adjustment is to increase or decrease the flow of CO2 into the reactor. The second adjustment is to increase or decrease the effluent rate. If you need to increase the amount of calcium being delivered to your tank, you would either increase the flow of CO2 (causing the pH to drop lower, dissolving the media quicker) OR decrease the effluent rate to allow longer contact time with the media. If you need to decrease the amount of calcium being delivered to your tank you do just the opposite.
  3. When first adding a calcium reactor to your tank, start with the manufacturer's recommended effluent and CO2 bubble rates for your size aquarium. Test your levels over the course of the next few weeks and adjust the CO2 bubble or effluent rate until your tank's desired calcium levels are sustained.
  4. Under normal usage, a 5 lb CO2 bottle lasts 6-12 months; media lasts approximately 6 months before needing to be replaced. While the initial investment may be a high, the long-term costs are very inexpensive.
  5. Under the items needed I mentioned the use of a pH controller and at this time I would like to mention why this is a nice piece of equipment to use in combination with a calcium reactor. In a reef tank we are trying to maintain high pH levels (normally between 8.2-8.4). When using a calcium reactor we need to inject CO2 into the water to lower the pH to dissolve the media. Normally a well-balanced tank and a properly set up calcium reactor will have little to no effect on the pH of our tank. But if something becomes out of balance within the tank or there is some type of equipment failure that causes excessive amounts of CO2 to be injected into the tank, the pH in the tank could drop to dangerous levels. A pH controller monitors the pH level of the tank and can turn off the flow of CO2 by shutting off power to the solenoid on the regulator if the pH level drops below a set point.
Kalkwasser (Kalk) Reactors

Kalkwasser Reactors What You'll Need
The reactor itself, kalkwasser (Calcium Hydroxide), reverse osmosis (RO) water or reverse osmosis/deionized (RO/DI) water, any tubing not included with the reactor for feeding water into and out of the reactor, digital timer (depending on the reactor type) and a dosing pump (optional).

The Basics: How Kalkwasser Reactors Work
Before I get into how they work, I want to explain what kalkwasser is. The word kalkwasser (shortened to kalk most of the time) is a German word for “limewater.” When calcium hydroxide is mixed with RO or RO/DI water it creates a solution that is rich in calcium, carbonate and also has a very high pH, normally above 12. This mixture can be very beneficial to a reef aquarium as it can not only supply calcium and carbonate to the tank but can also help bump up the pH, a problem many hobbyists contend with.

Since the mixture has a very high pH, you don't want to dump too much too quickly into your tank or you may raise the pH too high and cause major problems for your tank's inhabitants. Dripping the solution into the tank is the best method for delivery.

Now let's discuss how these reactors work. The reactor itself is generally comprised of an acrylic tube with some type of stirring device to help dissolve the kalkwasser in the water. Some reactors use a powerhead to mix the water while others use a stirring stick (acrylic rod). Both are efficient ways to accomplish the same task. RO or RO/DI water is fed directly into the reactor and dry kalkwasser powder is manually added; the stirring device helps dissolve the kalkwasser.

There are a couple of different methods for delivering the kalkwasser into the tank. One method is to use a dosing pump to drip the kalkwasser into the sump. The second method is to have the reactor gravity feed (drip) the solution into the sump. The water flow comes from a freshwater reservoir filled by your RO or RO/DI filter. The water travels through a dosing pump (or is gravity fed) into the reactor. The water then passes through the reactor and drips into your sump, delivering calcium-rich water to replenish what the tank's inhabitants have used. By pulling water from a freshwater reservoir, your reactor will be filled with water at all times. However, as the water drips out, the kalkwasser is depleted. You will therefore need to occasionally add more dry kalkwasser powder to the reactor.

A Few Additional Notes on Kalkwasser Reactors
The pH of kalkwasser is very high so you'll want to be careful not to overdose. Some people hook up their kalk reactors to their auto top-off systems. While in principle this is a great idea, you'll have to consider the potential mishaps. I have seen a few cases where top-off systems failed for one reason or another and too much kalkwasser was added to the aquarium. I find using a dosing pump to control the drip rate is one of the best ways to prevent this from occurring.

Some consider kalkwasser reactors to be the best way to keep up with a tank that has a very high demand for calcium (such as a heavily stocked tank filled with Acropora, Montipora and other stony corals and clams). Some use them in addition to a calcium reactor, while others use them instead of a calcium reactor.


Now that we have talked about reactors that add to the water, let's look at two reactors that remove organics from the tank. Nitrates and phosphates can cause major problems in a reef aquarium, like excessive algae growth (nuisance algae in particular) as well as inhibit the growth of corals by limiting their ability for calcification. Other dissolved organics can cause issues with water clarity. The use of filter media within a reactor can help alleviate these issues.

Nitrate Reactors

Nitrate Reactors What You'll Need
The reactor itself, media (if not included) and a feed pump (if needed or not included).

The Basics: How Nitrate Reactors Work
Water from the aquarium (with high nitrate levels) slowly flows through the reactor and bacteria forms to break down the nitrate. The area within the reactor becomes low in oxygen and the bacteria within it will use the oxygen found in the nitrate (NO3), thus reducing the nitrate levels. The filters become biologically active so it may take a few weeks for signs of lowered nitrates.

A Few Additional Notes on Nitrate Reactors
Since these reactors rely on bacteria to break down the nitrate, you'll need to give them time to perform. Once established, they will help reduce your nitrate levels. You'll just need the patience to allow the bacteria to form and do its job.

Most nitrate reactors are designed for large aquariums where high fish populations are the norm. While I have occasionally seen them used on smaller tanks, they are usually best on aquariums of 55 gallons or more.

Media Reactors

Media Reactors What You'll Need
The reactor itself, feed pump (if not included), filter media (such as a phosphate remover or carbon) and any tubing needed for hook up.

The Basics: How Media Reactors Work
These reactors are becoming increasingly popular, especially since the advent of Two Little Fishies' affordable Phosban Reactors. Now many companies produce similar reactors for use on all sizes of aquariums.

Water is fed into the reactor where it flows through the media (usually an upward flow) and then back into the tank or sump. By keeping the media fluidized (almost floating, if you will) within the reactor the water will come into contact with 100% of the media particles. This ensures the most effective use of the media within the reactor.

A Few Additional Notes on Media Reactors
Some people like to mix different media within the reactor, such as mixing carbon with phosphate media. However, in my experience, this is not the best option for a few reasons. First, the actual flow to keep the media fluid may differ. Carbon is usually heavier and requires a higher flow; most phosphate media is lighter and will require less flow. If they are mixed within the same canister, it is difficult to find an ideal flow rate that won't cause the lighter media to be pushed out of the reactor. Second, since the media will be “crashing” into each other, it can cause the softer media to break apart. This could cause that media to end up flowing out of the filter and into your tank. Lastly, the time frame in which they should be changed may be different. For example, the carbon may need to be replaced every 2-4 weeks, but the phosphate media may not need to be changed for 1-2 months. Once they are mixed together, you probably aren't going to want to try and separate them.

If you want to use two media types my suggestion would be to use two reactors, one for each type of media. You can then either run them on their own pump (one for each reactor) or use a Y hose barb fitting to split the outlet from a single pump into two outlets and run one to each reactor.

Closing Thoughts

I hope this overview of the most popular reactors available for your reef aquarium has given you some insight as to how they work and if they will be suitable for your tank. Calcium reactors, kalk reactors and nitrate reactors are generally best suited for larger tanks, but media reactors can be used on almost any sized aquarium. And, if you have any questions, please do not hesitate to let us know. Happy reefing!
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Approved Comments...
I am considering adding a media reactor and this gave me info on all of the different types of reactors available, I may not even need the "media" where as the carbon reactor would decrees or eliminate my need to add supplement calcium to my reef setup. Ty Approved: 10/3/2013
all encompassing with experiences and what to look out for - a plus for someone who doesnt know what could go wrong. Approved: 6/6/2009
Very organized and very informative. Thank You. Approved: 6/2/2009
Nice write up. I was not familiar at all with reactors and this explained very well I thought. Now I just need to save my money! Approved: 6/1/2009
I was unfamiliar with reactors in general. Good summary article. Approved: 5/31/2009
Came here to read about Zeo reactors, but nothing is mentioned. Approved: 5/31/2009
This is very helpful info presented in plain language and easy to understand. While I too would like more details presented in a more scientific manner, I do so because I have a better grasp of what the different reactors are for and how/what they do. Im a semi newbie at only a couple of years into this hobby and always appreciate clearly presented info to help me go forth in making better decisions about equipment, additives, tank mates, etc., etc. Approved: 5/30/2009
Since I have FW the information is interesting should I get the nerve up to go to SW, meanwhile I keep seeing ads for these devices and wondered what exactly they did. The only thing I would and is it would be useful to add foot-notes as to applicability in various environments such as FW or SW as the case may be. Approved: 5/30/2009
Im new to reefing and the amount of equipment available is overwhelming. I was almost to the point of forgetting the whole thing. I actually need a primer that simplifies the equipment I may need. Aquqrium stores seem to just want to sell the so called biggest and best (most expensive) This web site helps to add some reality to the hobby. Approved: 5/30/2009
very basic details so even the beginner could understand the concepts. Approved: 5/30/2009
Very well written and most helpful! Approved: 5/30/2009
spot on Approved: 5/30/2009
Needs info on Zeo reactors! Approved: 5/29/2009
Id love mroe scientific and engineering date. like choosing a proper flow rate and more detais on how the processes work... Approved: 5/29/2009
excellent material. good content, clear writing. and Ive been keeping reef tanks for 15 years. ESV Bi-ionic has worked well for me, but yes, a Kalk reactor rocks and kicks butt for my reef buddies. Approved: 5/29/2009
Didnt say what the p.h. is to be at in the Calcium reactor to dissolve the media. Ill tell you if I could do it all over I wouldnt buy a calcium reactor I would us the two part sollution. Its cheap and very simple and keeps the levels at where they need to be. I use soda ash in my auto top off and add calcium a couple times a week. Right now my calcium reactor isnt working because jbj co2 regulator isnt working. EDITOR'S NOTE: People run the pH levels differently, anywhere from around 6.2-7.2. A properly running calcium reactor takes away the chore of dosing chemical, hopefully you can get your regulator working and enjoy the ease of what a calcium reactor can do for you. Approved: 5/29/2009
Article Details
Created on 5/28/2009.
Last Modified on 6/1/2009.
Last Modified by Administrator.
Article has been viewed 14034 times.
Rated 9 out of 10 based on 61 votes.
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