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Pax Bellum ARID N18 Macroalgae Reactor - For Tanks 40-120 Gallons


Starting at $46/mo with . Learn more

Item #: 6U1418

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What’s a Pax Bellum A.R.I.D. N18 Reactor and how does it help my reef? Find out here!

This ingenious device utilizes macroalgae (specifically Chaetomorpha) to function as a nutrient export mechanism. The unique design of the ARID incorporates a coaxial LED light array housed in a tube to maximize light penetration and efficiency. Vapor heat pipe technology keeps the LEDs running cool for maximum light emission and lifespan while keeping the heat out of your aquarium. The LEDs were selected to supply intense PAR (photosynthetic active radiation) for rapid macroalgae growth. Maximum nutrient uptake is ensured by pushing system water evenly through the macroalgae mass.

Eliminate the clutter around your aquarium the ARID can function as a stand-alone filtration system. Designed to replace GFO (granulated ferric oxide), nitrate reactors, bio-pellet reactors, carbon dosing and even protein "skimmers" (foam fractionators). The ARID can perform functions the previously mentioned equipment can`t. Functions such as, increasing and stabilizing Ph from day to night, lowering dissolved CO2 concentrations, keeping dissolved oxygen even from day to night, and increasing dissolved oxygen to levels higher than can be attained with skimmers alone, all standard benefits of running the ARID system.

Small but mighty… The N-series (aka Nano series), N18 and N24 with its acrylic construction, are marketed for the aquarist that collects and displays their kit just as passionately as they collect and display the organisms residing within the main display. The aquarist can easily remove the light cover to reveal the inner workings of the opaque acrylic chamber and inspect the growth of the macro-algae inside. Highly polished acrylic edges are found throughout the N-series. The N24 has additional chamfered edges on the lid, flange, and base. The profile of the N24 mirrors that of the C-series while the N18 striving for the smallest envelope forgoes the recognizable ARID gear shaped profile for a more simple and compact form.

For the aquarist with a system from 40-120 US gallons (150-450 liters) the N18 is the smallest practical use of the ARID reactor embodied in a H.O.T. version. This model stands 16- 3/8” tall, and has a footprint of 6- ½” in diameter; the chamber and lid are produced entirely out of cast acrylic. The H.O.T. bracket is integral to the chamber flange and is adjustable for tank rims with bracing. The central inlet port on the bottom is ½” barb, as is the outlet on the lid. LED power consumption is 12 watts. Optional in-line quick-disconnects are available for the N18 separately. The N18 ships with starter solutions, 125ml of a 5% w/v solution of Nitrate+Molybdenum and 30ml of Iron+Manganese. To contain the blinding light emanating from within the reactor included is a magnetically fastened removable light blocking chamber cover.

Have a calcium reactor? Both the N18 and N24 can accept the same optional calcium injection assembly. This plumbing kit threads directly into the water inlet on the base of the N24 and placed inline with the supply tubing on the N18. The assembly and can be installed or removed at any time by the aquarist. This add-on kit functions by creating a high pressure ¼” push-fitting port and low pressure ¼" push-fitting port by separating the two ports by a user preset valve. The valve is a union that accepts restriction discs with different port sizes to create the desired back pressure to send water to a calcium reactor. The effluent coming from the calcium reactor passes through the check valve and drip counter included in the kit, then enters the ¼" push fitting port on the low pressure side of the calcium injection assembly. This is the port below the union containing the restriction disc. There the calcium reactor effluent mixes with the bulk of the water entering the ARID, and makes contact with the macro-algae where it is stripped of excess CO2 and phosphate before entering the reef system.

Do not use in conjunction with; carbon dosing, nitrate reactors, bio-pellet reactors, GFO (granulated ferric oxide)

Pax Bellum solutions should be used to run the ARID system successfully.

Recommended Dosage

Nitrate & Molybdenum
1ml raises 100L 0.5ppm
Does as needed to maintain NO3:PO4 ratio of approx. 100:1

Iron & Manganese
1 drop per 25 gallons daily.


  • System Size: 40-120 gallons aquariums
  • Body Material: Cast Acrylic with Tyvek cover (to keep light inside)
  • Total Height: 16.5"
  • Recommended Flow Rate: 150-200 gallons per hour
  • Recommended Pump: MJ900 Pump or equivalent
  • Total Footprint: 6.5"
  • Hose Inlet/Outlet: ½" FNPT hose (hose and pump not included)
  • LED Power Consumption: Standard 110v / 14 Watts
    • The supplied power supply can handle 220v input power with the use of a simple adapter cord to connect to the wall socket—mainly for European use


Can be placed within the sump if the tyvek wrap is kept above water. It is highly water resistant but submerging it is not recommended) As with any electrical device use caution and always unplug it when working in the sump area.

Manufacturer information

Pax Bellum LLC was founded in 2007 as a research and development company with a focus on saltwater life support systems. From its inception, the company explored a variety of ways to achieve maximum results balancing water chemistry within closed system hobby aquariums using algae. The ARID system and the technology behind it took 13+ years to develop and refine into the recognizable product it is today. Prototypes for the ARID were trialed extensively on multiple reef systems operated by well-respected and well-known professional aquarists. The first ARID units available for public consumption debuted at MACNA 2017 in New Orleans, Louisiana, and have been popular ever since. All Pax Bellum products are made in Vermont, USA.

5 Stars


Reviewed on
March 22, 2020

Yes, I would recommend this.

Great, but could use some improvements

Pros: Definitely works. Grows algae with no effort. Very compact and and dramatically improved my tank.

Cons: Really a pain to access to clean. Quick disconnects that specifically shut off flow are a necessity. Larger heat sink would also be beneficial.

This reactor is fantastic at growing algae. I am really surprised how well it grows cheato and how much this has reduced micro algae in my tank. I knocked it down a star because the maintenance is really a hassle. I purchased the separate disconnects from Pax Bellum. Those are really expensive and while they allow you to disconnect the tubing, they do not stop the flow of water as other quick disconnects do. So when you remove the reactor, you need to make sure there is not a siphon or that it will leak water everywhere. I'm surprised how hot the heat gets. It's very difficult to touch. So on the one had, it is clearly dissipating heat, but perhaps it needs to get bigger. Additionally, they recommend replacing the tubing around the light every 12 months. This is rather expensive and increases the cost of annual ownership substantially.

Still a nice reactor overall. Just expensive and requires thoughtful maintenance. Do not bury this in your cabinet or it will be a pain to clear.

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Reviewed on
February 20, 2020

Yes, I would recommend this.

PAX Bellum - very nice piece of equipment

Pros: The design of the reactor is well thought out. The design with the LED lights is very effective. The internal structure allows me to place algae pieces at top, middle and bottom. I purchased the quick connects also, which I believe, are necessary to be able to disconnect quickly, open the lid, pull out the algae, and put it back together in about 5 minutes.

Cons: This is not an all inclusive product. I had to purchase a pump, tubing and timer and clamps at the connectors to insure no water drips out. It would have been better if the product came with clear instructions on what all needed to be purchased. It took me 3 trips to the hardware store to get it all. Timers are usually not 3 pronged, so I had to return my first timer and find one that was a 3 prong timer. Those are all things that the manufacturer could have been more clear about - exact size of tubing, type of timer, pump, clamps etc. While the most expensive reactor in its group, I am hopeful the extra price will, in the long term, save me time on weekly maintenance.

I ordered this product after much research on what I needed to do to lower my nitrates. My nitrates had spiked to about 50, and after trying many things over many weeks, I decided the best solution is to add a component to my treatment plant. I researched more, and eventually came across the PAX Bellum. I had to gulp at the price - it was well over $100 more than the competitors, but the design of the reactor was what sold me. It took me a weekend to work through the kinks, largely my fault for not reading, and purchasing wrong size of tubing. But, once I got it figured out, it is really quite simple to install and operate. I can remove algae now with barely a drop of water getting on the floor. The reactor container fills up nicely over a week. There is no noise from the pump I purchased and all in all this works very well. So far, I love this new piece of equipment for its ease of use.

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Reviewed on
July 11, 2019

Yes, I would recommend this.

Excellent reactor

Pros: Excellent design, easy to clean, easy to install. The company has excellent support as well.

This is one of my best purchases for my Red Sea reefer 250. The design is excellent which i hang on the side of my tank. There is a logo opening on the side where you can see your algae growth. My nitrates and phosphates have been dramatically lower in conjunction with my protein skimmer. They recommend you do not use any phosphate or nitrate reactors since it will decrease nutrients for your algae growth. It grows alot of algae. The quick release valves are a must have since it stops water flow with a click and makes cleaning much easier. The plastic sleeve inserts makes cleaning a breeze which is done twice a month. Its takes me 5-10 minutes to clean from removing to reattaching and it grows alot of chaeto. Its the best design I have seen for this type of scrubber and a design you can be proud of instead of trying to hide in your cabinet. Highly recommended if you can get your hands on one.

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