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. ?