Art by Alexander Vasiljev, Copyright © 2015


Saturday, November 8, 2014


Reading through Goldfish Keepers Forum, I stumbled upon David Lains' post, where he referred to an article "Using Clay to Protect Catfish" published in Aquaculture North America. Being quite fascinated with this information, I searched the internet for more, and quickly became convinced of clay's undeniable benefits. Previously, I didn't know that clay had been in use for koi keeping for a very long time and is a standard practice for many koi breeders.

Of course not every clay is made equal. Out of several main clay groups, smectite group is of interest to us. It includes montmorillonite clay, also known as bentonite clay. Essentially, montmorillonite and bentonite clays are the same, with the first named in 1847 after a small town Montmorillon in western France, and the second after Benton Shale in Wyoming, USA in 1898. As montmorillonite clay was described and named first, it claims to be the valid name. This clay besides Montmorillon and Wyoming is mined in many other parts of the world and, depending on the deposit and location, have various mineral compositions. For the fish pond, however, only calcium montmorillonite clay is used.

Erosion formation in the soft bentonite clay, Cathedral Gorge State Park. Nevada, USA (open source image)

What are the benefits of using it in our fish tanks and ponds? The first main benefit of this clay is in its high mineral content that provides well over 50 macro, micro and trace elements "remineralizing" the aquatic system. All living organisms, including goldfish, require a constant supply of these elements for various biological processes to function normally. It also improves fish digestion. Second, is its detoxifying effect. The clay's particles are so small that the action of water molecules is enough to keep them in suspension or colloidal state. The surface area of particles in one gram of clay can reach almost 10,764 sq.ft (1000 sq.m). Negatively charged clay particles have the ability to "trap" or flocculate heavy metals, pesticides, free radicals and even virus cells, etc., making them easily removable with mechanical filters and water changes. Third, it accelerates bio filter work by positively affecting nitrifying bacteria, at the same time, based on the article mentioned above, it can protect the fish from certain pathogenic bacteria causing skin and gill tissue damage.

Properties of clay go well beyond described above. In fact, "montmorillonite is also known to cause micelles (lipid spheres) to assemble together into vesicles. These are structures that resemble cell membranes on many cells. It can also help nucleotides to assemble into RNA, which will end up inside the vesicles. It has been demonstrated that this could have generated highly complex RNA polymers that could reproduce the RNA trapped within the vesicles. This process may have led to the origin of life on Earth" (courtesy of Wikipedia). Interestingly, according to Old Testament, the first man was made of clay (earth, dust).

Now back to goldfish. There are several brands of calcium montmorillonite clay on the market for use in ponds - Ultimate Koi Clay (USA), Microbe-Lift CMC (USA), Terra Pond (USA), Thrive (USA), Kusuri Klay (UK) and Refresh (Japan) among a few others. Also, there is a human grade, edible calcium montmorillonite clay that is suitable for using in the fish tank, one of the known brands is Terramin. And yes, this clay is edible and is consumed as a food supplement by various animals, wild and domestic, including humans.

calcium montmorillonite clay

I started using calcium montmorillonite clay in my ranchu pond a month ago. Although, there are no obvious effects on my ranchu that I notice at the moment, I know that I am supplying important minerals and trace elements back into the system. In my case, municipal water is used, which based on published analysis contains only some of the necessary minerals. This, however, would'nt compare with the rich mineral content of calcium montmorillonite clay. Besides, the minerals must be in constant supply, as they are being used up by the living organisms.

I use half a teaspoon for my 100 gal pond once a week, by first "dissolving" clay in a bottle of pond water and then pouring it back into the pond. The water gets cloudy only for a few hours before it clears up, depending on the amount added. As I mentioned earlier, mineral composition of this clay greatly depends on the source. This is also true for pH and hardness that the clay will add to the pond water. Based on the tests provided in "Consumer Report: Koi Clay Mysteries Revealed" (see below) the pH of different clays ranges mostly from 7.65 to 8.7 and hardness from 31 to 74 ppm. In my case, added clay did not change the pH and it remained at 8.2. It is important, as in everything, not to overuse the clay and follow the recommended instructions. I would also recommend testing the water regularly, until it is clear how the clay effects the water environment. And as always, regular water changes are mandatory.

FURTHER READING: Consumer Report: Koi Clay Mysteries Revealed by Michael D.Cox, Koi Clay Article by Wayne Smith, Montmorillonite by Chris Neaves, Understanding Clay by Manky Sanke