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Activated Carbon and Phosphates

By Steven Pro

As readers might be aware from my previous round of testing of the Aquarium Systems/Marineland Labs hydrometers, product testing is becoming a passion of mine.  Frankly, it bothers me that I work in an industry that has so little in the way of proven fact and instead relies so heavily on anecdote.  Take the subject of my latest round of experiments, activated carbon.  I found one test of activated carbon and phosphates, but it was conducted almost ten years ago (Schiemer, 1997).  After searching the various message boards, it would seem that most people were simply using a particular brand of carbon based on nothing more than a recommendation from a fellow hobbyist.  To paraphrase, ‘I use so and so and it seems to work good enough for me.’  Is that really the best we can do?  I think not.

Marine and reef hobbyists are generally a rather well educated and informed group and yet we also seem to be bit lazy and careless preferring to rely on anecdote and random, uncontrolled use of various products teamed with simple observation.  But just because one adds something and later observes something else, does not prove a cause and effect relationship.  That is where a true controlled experiment is needed.

The first thing I had to do was buy a variety of carbons to test.  I went around to the local fish stores buying different samples of whatever they had on the shelf.  I then mail-ordered a few other brands to include in the testing.  In the end, I had accumulated eight different brands to test (listed in alphabetical order):

  • Aquarium Pharmaceuticals Activated Filter Carbon

  • Coralife Pelletized Activated Carbon

  • ESV Granular Activated Carbon

  • Kent Marine Reef Carbon

  • HBH Aqua-Pure High Density Carbon

  • Marineland Black Diamond

  • Seachem Matrix Carbon

  • Two Little Fishies HydroCarbon2 Granular Activated Carbon

I removed 50 ml of each carbon and placed that into a 500 ml beaker.  None of the samples of activated carbon were rinsed or pretreated in any way.  All produced a lot of black dust.  To that, I add 250 ml of deionized water that was run through a 0.5 micron sediment/carbon block filter and an Aquatechnik Kati-Ani separate resin deionizer.  The water produced was tested for phosphates prior to use and none were found according to a Salifert phosphate test kit.  The DI water also had a TDS reading of 2.  Each sample was then covered and agitated for 30 seconds before being allowed to sit for 24 hours.  After which, I tested the water samples again for any phosphate again using the Salifert brand test kit.  The results of the testing are in the table below.

Brand Tested

Phosphate Reading in ppm

Aquarium Pharmaceuticals Activated Filter Carbon


Coralife Pelletized Activated Carbon


ESV Granular Activated Carbon


Kent Marine Reef Carbon


HBH Aqua-Pure High Density Carbon


Marineland Black Diamond


Seachem Matrix Carbon


Two Little Fishies HydroCarbon2 Granular Activated Carbon


Deionized Water Control


The Salifert test kit is a colorimetric test that goes from clear to ever increasing shades of blue.  The Coralife and Two Little Fishies brand carbons both tested completely clear, therefore no traces of phosphate.  The Kent Marine and Seachem samples both had just a mild hint of blue to them , which corresponded to the lowest color match on the Salifert scale, 0.03 ppm phosphate.  ESV was a slightly darker blue color, and therefore a higher phosphate level of 0.25 ppm according to the color chart.  Aquarium Pharmaceuticals, HBH, and Marineland all had a comparatively strong blue color which matched 1 ppm phosphate on the Salifert scale.

It is important to note that my results vary dramatically from the testing done by Gregory Schiemer in 1997.  His experiment found no trace of phosphate in any sample tested.  The editors of Mr. Schiemer’s article stated that no “manufacturer” actually manufacturers the carbon they sell.  They simply purchase bulk activated carbon and label it for resale.  I cannot dispute those claims and have every reason to believe they are accurate.  It is quite possible that in the years since Mr. Schiemer’s testing, the sources for activated carbon have changed and some brands are no longer phosphate-free.

It should also be noted that I only used one sample of each brand.  There could have just have easily been some sort of quality control issue which permitted a tainted sample of activated carbon to get through for sale.

Regardless, based on my testing, I would encourage all hobbyists to repeat this simple experiment with their own activated carbon prior to use.  In this manner, they can be assured of not importing additional phosphate and can also add to the body of knowledge on the various brands of carbon by reporting their results.


Schiemer, Gregory. “About Activated Carbon” Aquarium Frontiers Online Magazine, July 1997.

Related Reading:

Harker, Richard. “Granular Activated Carbon In The Reef Tank: Fact, Folklore And Its Effectiveness In Removing Gelbstoff - Part One” Aquarium Frontiers Online Magazine, May 1998.

Harker, Richard. “Granular Activated Carbon In The Reef Tank: Fact, Folklore And Its Effectiveness In Removing Gelbstoff - Part Two” Aquarium Frontiers Online Magazine, June 1998.

"Activated Carbon" by Dr. Timothy Hovanec

"Revisiting Activated Carbon" by Dr. Timothy Hovanec

Chemical Filtration, Including Carbon & Phosphates

Selection and Placement of Activated Carbon in Marine Aquaria by Adam Jenkins, Chemical Filtration by Bob Fenner & FAQs on: Chemical Filtrants (e.g. Polyfilter, Chemipure, Purigen), Chemical Filtrants 2, Carbon, Carbon 2, Rationale, Types/Qualities/Selection, Sources, Placement, Renewal, Negative Reactions,

Phosphates in Marine Aquarium Systems by Marco Lichtenberger, & FAQs, FAQs 2, & FAQs on Phosphate: Importance, Science, Measure, Sources, Control, Chemical Filtrants, Troubleshooting/Fixing,


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