Substrate Additives

To most of us the idea of adding “dirt” to our aquariums seems ludicrous, but that is just what some aquatic plants need. There has been some debate of whether aquatic plants extract nutrients through their roots or through their leaves. One camp claims the roots are for securing the plant down and nutrient absorption is through the leaves. Another camp claims that leaves are for photosynthesis and the roots absorb nutrients from the soil. I believe aquatic plants can do both, but may rely on one means more than another and that varies from plant to plant. When I pull a nice Amazon sword or Cryptocoryne from my tank and look at the extensive root system, I can’t believe that it is just for securing the plant down. But, when the Amazon swords look chlorotic and I add chelated iron to the water, their leaves return to a nice green shade. That leads me to believe that the iron was absorbed through the leaves, not the roots.

Horst and Kipper, in The Optimum Aquarium, conducted experiments with different substrate types and demonstrated that Cryptocoryne wendtii grew much better in gravel containing iron additives. That is where the “dirt” comes in. In the tropics, where most of our aquatic plants come from, there is an abundance of a clay soil rich in iron and manganese called laterite. The iron content is so high that the soil is red.

In the aquarium, laterite serves two purposes. First it is a source of micronutrients that the plants can absorb through their roots. Second, it provides chelating sites for micronutrients, ensuring that they are at the correct valence (e.g. iron stays as Fe2+ instead of Fe3+). For the second purpose we need a gentle current through the substrate (see HAPpenings in the October Calquarium).

The idea is that the first 1/3 of the substrate be composed of a mix of laterite and sand or gravel. The proportions vary depending on the brand. I decided on how much laterite I was going to add and mixed with 1/3 of the sand I was going to use. The other 2/3 of the substrate should be clean sand or gravel.

Unfortunately laterite is hard to find in Calgary. The few times I looked for it at the aquarium shops I got odd looks followed by the question “you want what?”. Oddly enough, I have seen two types while I was shopping for other things. Aquarium Products sells laterite cubes, roughly 1 cm each way. A packet of 10 cost me $5.99 (definitely not dirt cheap). A German company, I can’t remember the name, also sold laterite in a larger container for about $20.00, which is more reasonable. Don’t rely on the availability of either one, for I haven’t seen them for a while. When I set up my large planted tank in 1995 I had to order laterite from the Mail Order Petshop. For about $30.00, freight included, I got two containers (about 900 g) of Thiel Aquatec laterite, supposedly enough for a 1000-liter aquarium. I still remember my wife calling me at work to tell me that my dirt had arrived!

The Aquarium Products laterite definitely works. I had a problem with my Amazon swordplants shedding their outer leaves for no apparent reason. The plants looked fine, but were shrinking slowly as they couldn’t produce enough new leaves to keep up with the losses. I added one cube per plant, right under the roots, and the shedding stopped. The Cryptocorynes grew a little better also. One side effect is that this particular brand of laterite tints the water to a pinkish shade, even when covered by gravel or sand and there is no substrate circulation. The six cubes I used in my 60-liter tank turned the water to a deep burgundy color for a few weeks. Regular water changes took care of the coloration. The fish weren’t bothered by it.

The Thiel laterite was used proactively in my large tank. There was so much driftwood in the tank that I couldn’t tell if the laterite colored the water. For a few weeks it looked like I had filled my tank with tea. The plants grew very well for the first few months, then started to look chlorotic. Additions of chelated iron stopped the chlorosis, but I thought the laterite would provide the necessary iron. This particular brand of laterite is not reddish as I expected, it is light brown and is mined in the USA not the tropics.

I should say that both brands of laterite behave very well. Whenever I pull out a plant, any laterite that comes to the surface tends to settle quickly and there is hardly any clouding of the water.

It is possible that our local clay may act like tropical laterite by providing chelating sites. Judging from the color of the clay around here and the iron content of our water, I would suspect that the clay is low in iron. I was very tempted to do some laterite mining while I was in Brazil in 1996. Everywhere I looked were tonnes of clean looking red laterite. Nobody was going to miss a kilo or so, and if I baked it in the oven it would be sterilized. The thoughts of explaining to my grandmother why there was “dirt” in the oven, and then explaining to the customs officials why I was bringing soil back to Canada changed my mind.

There are other substrate additives one can use. Potting soil and top soil are two of them. Unfortunately both are very messy and don’t behave well like laterite. All the “fluffyfiers” added to potting soil will try to keep it in suspension, plus any organic debris (leaves, roots) will tend to rot anaerobically. Also the soil can’t have any fertilizers unless you don’t mind dead fish and lush algal growth.

Peat moss is also used as a substrate additive for its chelating properties. The only experience I’ve had with it was trying to get a few pieces waterlogged in a sealed container. The pieces got waterlogged but when I opened the container the smell of hydrogen sulfide was terrible, so I threw the whole thing away. Sera Aquaristik markets a special peat moss additive that is fertilized. Their claim is that it is better than laterite, though I haven’t tried it or read any accounts of its use.

Regardless of what additive is used, its effectiveness ends after a few years. The micronutrients are used up by the plants and their growth is slowed down. Then it is time to tear down the substrate and start all over again with fresh additives. For a small tank that is not too bad, but I don’t even want to think about tearing down my large tank. At times like this I can’t help thinking about my friend’s tank with the giant Amazon swordplants. He doesn’t use substrate additives, just large gravel and fish droppings with CO2 and iron fertilization. After a couple of years his plants still look great, so it looks like there is an easier way to have nice plants and fish. Anybody out there with time and space to try setting up two tanks, one in The Optimum Aquarium’s way, and the other like my friend’s and compare them side by side? I think it would be a worthwhile experiment.



Horst, Kaspar, & Kipper, Horst E. (1986). The Optimum Aquarium. Bielefeld, Germany: AD aquadocumenta Verlag GmbH.

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