Category: Plants

How to keep, grow and propagate plants.

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.

Rec.aquaria newsgroup Plant FAQ?

Water Circulation Through the Substrate

In my last HAPpenings article I spoke about substrate choices for plants, now it is time to discuss something just as important: water circulation. In nature water is always on the move. We may not notice it, but it circulates in lakes that are apparently still, and through the soil also. Springs and riverbeds are two examples of water that is moving through the soil.

So why should one care about this kind of circulation in an aquarium? If you recall my last article I mentioned that certain combinations of substrate will go anaerobic without water circulation. More importantly substrate water circulation can provide a more even micronutrient fertilization and denitrification (the removal of nitrates from the water). The result is better plant growth.


Plants have an easier time absorbing ammonium (NH4+) than nitrate (NO3), and in this aspect they are better off if we didn’t use our filters and reduced aeration. Also the micronutrients (mostly metal ions) come out of solution when oxidized and are of little use to the plants. If these oxidized micronutrients are reduced (have their oxygen removed) they can go back into solution and be used by the plants.

One can achieve this reduction effect by setting up a very slow water current through the substrate. As the water slowly flows through the substrate, plant roots and the anaerobic conditions there perform the desired reduction on the oxidized micronutrients. Likewise, anaerobic bacteria reduce the nitrate to nitrogen via a series of other nitrogen compounds. The nitrogen finally escapes as gas bubbles. The key word here is slow. We want to circulate the volume water in the substrate once over the whole day. Any faster and the substrate will become too aerated for the process to work.


Since the water flow must be very slow, undergravel filters are out of the picture. What is needed here is substrate heating. The heated water rises up, bringing colder water down. The easiest and safest ways to achieve this is through the use either heating cables or heating pipes under the substrate. The spacing between the cables or pipes provide a way for the cold water to sink, while the warm water rises in the immediate periphery of the heating medium.

Aquarium heating cables are hard to find in North America, and are reputed to be very expensive when available. You can find references to them in just about any European aquarium literature though. Luckily for the handy ones among us, they are not too difficult to make. All one needs are wires, a transformer, some fuses, a timer, and a background in electrical engineering. Yes, I’m serious, they are simple once you know what to do.

Be aware that water and electricity are a dangerous mix. So I will not explain how to build the cables here. Instead I will point you to the references for a very good article, written by a computer scientist and an electrical engineer, on how to make them. The one thing I found incorrect in the article is that Radio Shack, in Calgary at least, does not carry wires in the required gauges. I purchased the wires from a more specialized electronics shop along with the transformer.

If you are not comfortable with electricity, there is an alternative: piped hot water. Basically you can set up a closed system with an aquarium heater, a powerhead, and thin pipes to heat your aquarium’s substrate. See the references for another good article written, interestingly enough, by two electrical engineers. This set up is more bulky than a wire one, but it has the advantage that its power can be changed by changing the heater. The electrical setup relies on the different resistance values in the different wire gauges matched with an appropriate transformer, so it is not simple to change the power output.


So, is any of this worth the trouble? I don’t know. The theory makes sense. Both “how to” articles mention great plant growth as a result of the substrate heating. My plants do great in my 150cm tank with substrate heating, but it is only one factor of many. I haven’t conducted a test to isolate substrate heating and see what the effect really is.

There are a couple of things I have observed though. Measurements for nitrate and nitrite in the tank have consistently been in the lowest measurable end of the scale. I have an established trickle filter in that tank that should be converting ammonium into nitrate quite efficiently. I don’t think the plants are using the ammonium faster than the filter can convert it to nitrate, so it is possible that the substrate is acting as a denitrification medium. I can’t verify if there is an effect on micronutrients.

Not surprisingly, the tank does not need a conventional aquarium heater. The heating cables alone can keep the tank’s temperature at 21C when the room is at 19C. Additional heat from the pump and the lights push the temperature to about 25C. The buried cables are therefore an effective aquarium heater that won’t overheat the tank, is out of reach of fish (no burns), and is inconspicuous. Considering the parts cost me about $50.00, and substrate heating is part of the Optimum Aquarium, I chose to try it out lest it be a limiting factor for plant growth. But adding substrate heating to a tank that is already set up is not my idea of fun.

Should anyone try substrate heating, let me know about your results. Good luck!


Gesting, Berti. (1991). “Advanced Water Management for Tropical Freshwater Aquaria (Part 2).” Aquarist and Pondkeeper, October: pp. 26-28.

Hamilton, Earle, & Smelt, Phillip (1995). “Hot Gravel. Build a Low Cost Substrate Heater for Planted Tanks.” Aquarium Fish Magazine, July: pp. 38-54.

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

Resler, Dan, & Behle, Uwe. (1995). “Heat from Cables. Do-It-Yourself Substrate Heating Cables.” Aquarium Fish Magazine, March: pp. 66-77.

Scheurmann, Ines. (1985). The New Aquarium Handbook. Woodbury, NY: Barron’s Educational Series Inc. ?

A Question Of Substrate

A few people have asked me what I use as a substrate in my plant tanks, and what is the best substrate for healthy plant growth. After trying a few different options and seeing how other people grow their plants I’ve come to the conclusion that just about anything will do. It comes down to taste, the type of fish kept in the tank, availability, cost, and the amount of maintenance required.

In an aquarium there really isn’t much water circulation through the substrate unless we use undergravel filters or substrate heating. It is this circulation that keeps the soil aerated. Without it, the soil could run out of oxygen (become anaerobic) and turn black. Organic debris decomposing in this environment would ferment and release hydrogen sulfide (that noxious rotten egg smell). This is just what we don’t want happening in our tanks. And so we can not use just any natural substrate for our aquarium plants. In nature aquatic plants grow in gravel, sand, clay, and a mixture of roots and decomposing matter. But let’s assume we don’t have any substrate circulation in our aquaria. In such a situation, very fine sand and a mixture of roots and decomposing matter will definitely go anaerobic. Clay isn’t a good choice either because any water movement near the bottom would stir it up or cloud the water, although clay can be used if it is topped with a layer of sand or gravel. This leaves us with different grades of sand and gravel.

Now that we have narrowed our choices, what should one use? First, one must make sure that the material won’t react with the water. It must not dissolve or leach anything. A few drops of vinegar or lemon juice will indicate if the material contains a carbonate by producing bubbles. Carbonates should not be used because they dissolve and make the water hard and alkaline (fine for East African and Central American cichlids, but not the best for plants). Also, the material should not have sharp edges as these are abrasive to fish, plant roots, and the aquarist’s hands.

Secondly, one has to decide what grade of sand or gravel to use. Horst and Kipper (1986) recommend using gravel 2 mm to 3 mm in diameter. Scheurmann (1985) recommends using sand 1 mm to 2 mm in diameter. From personal experience, I’ve had success with both sand (mixed grade ranging from less than 1 mm to 2 mm diameter) and red flint of 2 mm to 4 mm in diameter. Some club members are very successful using gravel with grain sizes of 5 mm and larger. I find that roots grow finer and more extensively when sand is used. One thing to avoid for sure is sand with grains consistently in the 1 mm and less range. Such sand tends to compact and prevents roots from growing; it also becomes anaerobic very easily.

For aquarium purposes, sand and gravel differ in many ways beside their relative grain size. Here I will define sand as having grain sizes of 2 mm and smaller and gravel as having grains 2 mm and larger. When added to water the two behave differently. Sand packs together and stays soft if the majority of the grains are on the larger side, or packs solidly if the majority of the grains are on the smaller side. Gravel, on the other hand, stays loose with gaps between the grains. The larger the grain size, the larger the gap size.

When it comes to setting up, sand demands some special attention and creativity. It holds more dust when dry, so it takes longer to wash. A standard undergravel filter won’t work with sand, and perish the thought of sand being sucked by a powerhead! I have read on the Internet of an aquarist who wrapped fabric on the undergravel filter plates and used it successfully, though reverse flow may be out of the question. As for powerheads, just keep the intakes a few centimeters higher than the sand surface. A stone under the intake will keep sand from being sucked in. If you hear grinding noises from the powerhead, dismantle it and clean it thoroughly. Sand in powerheads is not necessarily fatal, but you don’t want it going on for any length of time.

Cleaning the aquarium is easier with sand. Because sand packs fairly tightly, debris stays on top of it. Therefore it is not necessary to “vacuum” sand as one would do with gravel. A quick pass with a siphon hose is enough to suck up the debris with a little sand. When finished, just wash whatever sand was sucked out and put it back in the aquarium.

Setting up with gravel is straight forward, and gravel is less likely to get sucked into filter intakes. With the larger sizes of gravel, vacuuming becomes important. All those gaps between the grains will fill up with debris and the substrate could become anaerobic. Plants with heavy root growth will help here, but one must make sure that open areas are clean of debris. Don’t vacuum too much around plants as the debris provides nutrition to them.

Sand is available at aquarium shops and as a construction material from hardware and landscaping stores. The selection is not great. The most common is a smooth quartz sand, beige in color, with wildly varying grain sizes (from dust to just over 2 mm). I have also seen for sale a white carbonate sand, which is not suitable for our purposes here. The effect of the lighter color of sand on the fishes’ colors can be offset by heavy planting and a dark background. Sand is very cheap at hardware and landscaping stores. A 25 kg bag of “play sand” (enough for two 15-gallon tanks) costs less than $3.00. Unfortunately you never know what variation in grain sizes you will end up with (lately it has been too fine). Sandblasting sand is more expensive, but the grain size should be more consistent.

Gravel is much more diverse in its availability. It comes in all colors of the rainbow, black, white, and anything in between. Some types of gravel are smooth, others look like broken glass. I’ve had some black gravel that was actually clear gravel with a black coating that would wear off slowly. Look for something that has a natural look to it and is not too coarse. Darker gravel makes fish look darker and more colorful. The flip side of gravel is its cost. Most of it is only available in smaller quantities at aquarium stores, so costs are higher. Natural gravel is available at construction supply stores at more reasonable prices, but you could end up with a mixture of rock types, some being carbonates.

When it comes to fish, any fish that burrows or eats from the bottom will love sand. Eels, loaches, and whip-tailed or banjo catfishes will bury themselves in sand. Corydoras will dig in it most of the day and their barbels will stay in great shape. It is advisable to have fish that will sift the sand because the top layer may develop a film of algae and become encrusted; constant sifting will keep the sand loose. Other fish don’t seem to care if the bottom is sand or gravel.

Bottom-dwelling fish that come from areas with strong currents prefer gravel or even rocks. Burrowing fish won’t do as well in gravel, especially if it is coarse or has sharp edges. I’ve seen Corydoras without lips, much less barbels, in tanks with round-grained gravel but with a coarse texture.

For the record, I have used red flint in my tanks and had an underwater jungle growing in my 60-liter tank. I liked the color and texture of the gravel, but not the price. I used sand when I set up my large tank and now I will not go back to gravel. All my tanks have sand in them now. I like the look and price of sand, and most importantly my plants and fish are thriving.

In conclusion, the choice of substrate is really yours to make. This article hopefully provided you with some useful information. I will discuss substrate additives and water circulation in future articles.


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

Scheurmann, Ines. (1985). The New Aquarium Handbook. Woodbury, NY: Barron’s Educational Series Inc.?