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Pond Magnet Foolishness

There is no science-based reason why you should buy magnets for the pipes in your pond, despite the sales pitches.  

by Brian Dunning

Filed under Consumer Ripoffs, Paranormal

Skeptoid Podcast #7
November 14, 2006
Podcast transcript | Download | Subscribe
Also available in Japanese | Russian



With a net in one hand and a pH testing kit in the other, let's wade into the murky waters of pond chemistry to test the latest fad in koi pond maintenance: magnets.

I was visiting my cousin up in Portland Oregon, and he showed me his cool koi pond. Being a koi pond guy myself, we compared notes on filter equipment, water testing, plant types, and all the usual stuff. His main filter pipe had a group of powerful magnets arrayed around it, which was something I hadn't seen before. I'm no super expert on ponds, so I guessed that maybe it was doing something useful like grabbing out metal filings from the pump. I asked him what the magnets were for, and he wasn't sure, but it was something to do with water chemistry. Right away, my radar went up. Unless there were significant amounts of iron, nickel, or cobalt in his water (those are the only three elements that are ferromagnetic) that required being magnetically held against the side of the pipe, there's really no physical way for the magnets to have any effect on anything in the water.

My cousin's friend at the pond store had recommended that he install the magnets, and he'd followed her expert recommendation. After all, he had no reason to doubt her suggestions. At my urging, he called her up to ask what the heck the magnets were supposed to do. She hemmed and hawed, said something about water clarity or chemistry or algae, and finally confessed that she had no idea, and that it was just a standard thing that a lot of pond owners do. The magnets were pretty expensive, so it wasn't surprising that a pond store would push them.

So I turned to the Internet, as I often do in times of need. It didn't take much searching to find the standard claim about magnets and ponds, and it has to do with algae growth. The claim is that magnets, mounted inline along any of the pipes, improve water clarity by altering iron alignment in free-floating algae, thus inhibiting photosynthesis. I also found one or two references to reducing lime scale build-up inside the pipes, but since this claim didn't even pretend to suggest a mechanism that might produce this effect, I discounted it. Lime scale is calcium carbonate. It contains none of the three ferromagnetic elements and is thus completely unaffected by magnets. That claim pretty much busts itself, no help from me needed.

So what about this reduction of algae? The claim is that the algae will be reduced because its photosynthesis will be inhibited, due to the realignment of its iron. This is a fairly common type of claim. It makes no sense; but because it uses common scientific-sounding words, many people will simply accept it at face value without questioning it. My cousin's friend at the pond store did, and when she repeated it to my cousin, he did too. I even accepted it when he told me, albeit tentatively, pending some kind of reasonable explanation.

Here are the two problems with this claim. Number one, photosynthesis is a chemical reaction among carbohydrates. Iron is not involved. The presence of iron would neither hinder nor help photosynthesis, so far as I've been able to find. The magnetic orientation of any iron molecules nearby is not relevant. Magnetically realigning nearby iron has no effect on photosynthesis, and will not harm a plant or algae in any way. Number two, iron, which is found in human blood hemoglobin, is not present in chlorophyll or in the other proteins involved in plant photosynthesis. Although I've never spent the time to wave a magnet past a plant several times a day, I'd be awfully surprised if that plant's photosynthesis stopped and it died as a result.

Another problem with this claim is the concept that briefly passing a non-magnetic object through a magnetic field will leave it altered after the magnet is removed. This is like turning the light in a room on then off again, and expecting the furniture to be somehow residually contaminated with light. Electromagnetic radiation doesn't work that way.

I should mention that when I set out to research this claim, I didn't merely gather enough information to shoot the claim down and then quit. I did make a good effort to find research supporting the effects of magnets on algae. But, since there are no plausible claims, there has never really been anything for anyone to test. However, I did find something close. In 2005, the Woods Hole Oceanographic Institute announced the results of research they'd done on certain bacteria that are known to carry magnetic crystals. These are called magnetotactic bacteria. In 1970, magnetotactic bacteria were also discovered in the Southern Hemisphere whose magnetic crystals were flipped around. The purpose of these tiny internal compasses has never been known, but since the 1970 discovery, the working hypothesis has been that they use the compasses to help navigate either up or down to find water with the best oxygen concentration. This would be consistent with the need for the polarity to be reversed in the Southern Hemisphere.

Alas for the pond magnet manufacturers, Woods Hole's research found that north-polarity and south-polarity bacteria are both found intermixed in both hemispheres, and also that there are numerous individuals who lack the crystals completely. All three types of bacteria navigate equally well to the water depths with the most desirable oxygen levels. The conclusion of the research is that the purpose of the magnetic crystals remains unknown, but it's clear that its reversal or even its total lack makes no difference to the health or life cycle of the bacteria. And, once the bar magnet was removed from the microscope slide, the magnetotactic bacteria realigned themselves normally with the earth's magnetic field, according to the polarity of each. There were no residual effects of having been briefly placed near a magnet.

So save your money if you own a koi pond or other aquarium, and don't buy any magnetic water-health gizmos. I recommend that you do your own research, or at least ask for a reasonable explanation, whenever any salesman offers you a product that claims to accomplish something far fetched or contrary to your understanding of the laws of nature.

By Brian Dunning

Please contact us with any corrections or feedback.


Cite this article:
Dunning, B. "Pond Magnet Foolishness." Skeptoid Podcast. Skeptoid Media, 14 Nov 2006. Web. 22 Oct 2017. <>


References & Further Reading

Blakemore, R., Frankel, R., Kalmijn, A. "South-seeking magnetotactic bacteria in the Southern Hemisphere." Nature. 24 Jul. 1980, Volume 286, Number 5771: 384-385.

Buttner, J., Soderberg, R., Terlizzi, D. "An Introduction to Water Chemistry in Freshwater Aquaculture." The Aquaculture Network Information Center (AquaNIC). The Aquaculture Network Information Center (AquaNIC), 1 Jan. 1993. Web. 14 Nov. 2006. <>

Fletcher, N. The Ultimate Koi. Lydney, Gloucestershire: Ringpress Books Ltd, 1999. 168.

Murata, N. "Effect of magnetism on the growth of Dunaliella Salina." Research in photosynthesis: proceedings of the IXth International Congress. 30 Aug. 1992, Volume III: 87-90.

Schüler, D. Magnetoreception and Magnetosomes in Bacteria. New York: Springer-Verlag, 2007. 2-5.

Skomal, G. Freshwater Aquarium: Your Happy Healthy Pet. Hoboken, NJ: Wiley Publishing, 2005. 43.


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