Short Take: Calcium Reactor Substrate — Phosphate Levels

In an endeavor to save a few dollars in this very expensive hobby, several years back I tested some alternative calcium reactor substrates 1 to determine their elemental profiles using an analytical technique known as inductively coupled plasma. Prior to my work, Craig Bingman also tested some calcium reactor substrates 2 in a similar fashion. In a nutshell, from a phosphorus standpoint Dr. Bingman’s experiments suggested that Super Calc Gold (which from what I can tell is no longer commercially available) had about twice the concentration of phosphate as Koralith (sometimes also called Hydro-Carbonate). In my experiments I found that a locally quarried limestone had phosphate levels several fold higher than Koralith, but still about 40% lower than a crushed coral aragonite that I’d been using in my calcium reactors at the time.

Since the time of my experiments, a new calcium reactor substrate called ARM (aragonite reactor media) has been marketed by CaribSea. CaribSea was recently claiming in advertisements in some of the aquarium literature3 that according to testing conducted by an independent lab, the phosphate content of ARM was less than 1/500th of that in Koralith medium. As folks in the sciences know, exceptional claims demand exceptional experimental proof. I was a bit skeptical to say the least. In looking at ARM even with the naked eye, it is clear that it is made up of pulverized corals, clam and snail shells, and other naturally occurring calcium carbonate materials of marine origin, and like all such materials significant quantities of phosphate would have to be present as they are in all life forms.


Testing methodology

  1. A 10 gram sample of each calcium reactor substrate was dissolved in concentrated lab grade hydrochloric acid in about 300 ml of DI water. I used 25 ml concentrated hydrochloric acid, which was a slight excess.
  2. The samples were neutralized back to a pH of about 7.9 with 5 molar sodium hydroxide (took about 24 ml).
  3. Samples were then diluted to a total of 1 liter with DI water.
  4. The resulting solutions were allowed to settle for a few moments, and then samples were taken and run on the Hach phosphate test kit (model PO-19). To get into the range of the Hach test kit (0-50 mg/l) it was necessary for me to further dilute the sample from my locally quarried limestone.
  5. As a secondary test, I further diluted all the samples by 10X and ran the Hach calcium test kit (model HA-4P) to see if the same amount of calcium was being generated by the dissolution of the substrates, and to allow a calculation of a Ca/PO4 ratio for each substrate.





A. ARM Reactor Substrate

The phosphate in the ARM reactor medium sample (received from Champion Lighting) tested out to 2.0 ppm. ARM calcium came out at 385 ppm, or 3850 ppm (when accounting for dilution). Therefore, the calcium to phosphate (Ca/PO4) ratio for ARM was 1925 (high numbers are best).


B. Koralith Reactor Substrate

Koralith medium (received from Champion Lighting) came out at 2.8 ppm. Koralith calcium tested out at 385 ppm, or 3850 ppm (when accounting for dilution). Therefore, the Ca/PO4 ratio for Koralith was 1375. Koralith therefore has 40% higher phosphate levels than ARM.


C. Locally collect limestone

My locally collected limestone (Conklin Limestone Quarry, Lincoln, RI) came out at 6 ppm. The calcium level came out a bit low at 300 ppm, or 3000 ppm (when accounting for dilution). It is possible that this sample was not completely dissolved, which would therefore affect the phosphate number, but not the Ca/PO4. The Ca/PO4 ratio for the limestone was 500.


D. Reagent Controls

A control of 25 ml concentrated hydrochloric acid neutralized with the sodium hydroxide and diluted as the other samples came up very low, but detectable using the Hach phosphate test kit, and the background had no effect over the numbers above. Because no calcium carbonate sample was dissolved in the processing of this sample, considerably more sodium hydroxide was needed to get the pH into the sampling range.



Conclusions and Discussion

As shown in the table, the phosphate level of ARM calcium reactor substrate is almost 30% lower than the level in Koralith, but certainly no where near the 1/500th number claimed by the manufacturer. The phosphate levels in the Conklin limestone are considerably higher, but still within a range that in my opinion makes it useable, and, since I have been using the Conklin limestone for about the past 2 years, I know it is usable!

SubstratePhosphate level (ppm)Ca/PO4 ratio (high numbers are best)
ARM from Carib Sea2.01925
Conklin Limestone>6.0500

How important is the level of phosphate in your calcium reactor substrate? Well, the details of that answer go well beyond the scope of this article, but have been discussed in detail in the reefkeeping literature.14,15 Suffice to say that significant quantities of phosphate are added to our tanks by simply feeding the fish. High levels of phosphate are thought to promote algae growth, and in some cases may inhibit normal calcification in corals.14 Because of this, any methods or procedures that limit the introduction of additional phosphate into a reef tank seem prudent. If you really think your corals and critter are growing so fast that they are limited by low levels of phosphate, it’s very easy to reach for that can of flake food!


Other Notes

The ARM medium appears to have a lot of shells in it. I have no idea what the trace nutrient profile looks like for this material. Unlike the other substrates that were tested, when dissolving the ARM medium, there was quite a lot of brown foam formed. I was careful to collapse this foam back down into the solution before taking samples for the analysis.

Before I performed the experiments in the lab, I did some quick and dirty measurements at home in my sink with some hardware store quality hydrochloric acid (also called Muriatic acid) and a Pinpoint pH probe. The sodium hydroxide I used was lab grade, but I would conjecture that one might be able to purchase sodium hydroxide from the hardware store of sufficient grade to make some home experimentation possible. Anyhow, my quick and dirty experiments at home yielded similar numbers, but there was a bit more background from the reagents. I would reason that if one just wanted to directly compare head to head any particular calcium reactor substrates for phosphate levels one could do it at home with hardware quality reagents. Please be very careful if you try this, the acids and bases use in these experiments are VERY potent, and capable of burning through skin and clothing, and some of the fumes given off could easily make one very sick.



Thanks to Randy Holmes-Farley and Habib Sekha for testing methodology suggestions.




  1. Alternative Calcium Reactor Substrates, by Greg Hiller–+Biochemistry+of+Aquaria&RecordNo=1571
  2. Calcium Carbonate for CaCO3/CO2 Reactors: More Than Meets the Eye, by Craig Bingman
  3. CaribSea Inc. advertisement for ARM calcium reactor medium, in December 2002 Freshwater and Marine Aquarium, pg. 91.
  4. Phosphate….What is it and why should you care, by Randy Holmes- Farley–+Biochemistry+of+Aquaria&RecordNo=2481
  5. Phosphorus: Algae’s Best Friend, by Randy Holmes-Farley
  Advanced Aquarist, Advanced Aquarist

 Greg Hiller

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