Substrate Heating in the Planted Aquarium: Myth, Magic or the Real Deal?

Copyright 2000, George and Karla Booth

Probably everyone that has kept and enjoyed a planted aquarium has had the following experience. We often visit fish stores just to see what is new and perhaps spot an unusual plant for our collection. Sometimes a store will have a stunning display tank that looks like it came straight out of an exotic jungle river. That store will go on our "A" list and we will make it a point to visit it more often. But when we go back in a few months, that same tank now resembles Lake Erie at its worst! What happened?

More than likely, the store had received a fresh shipment of plants and decided to spiff up their display tank. They assigned the "plant guru" sales associate to create a nice arrangement and perhaps set up a display of the products that contributed to the "success" of the tank. But they didn’t pay attention to the long-term needs of the plants and the plants began to suffer. We think this scenario is quite common, especially with people just starting out with live plants. It is not difficult to set up a beautiful planted tank and have it look nice for a short period. But how many aquarists can maintain a spectacular planted aquarium for 2, 3, 4, or 5 years without heroic efforts or starting over from scratch? In our experience, it is a rare aquarium that is stable over the long haul.

We have kept planted aquaria for over 14 years and have tried many methods in an effort to achieve long term stability. The one method that has worked the best for us is substrate heating. We first learned about substrate heating when reading "The Optimum Aquarium", published in English by Dupla in 1986. Of all the topics in the book, two major concepts were brand new to us – CO2 injection and substrate heating. CO2 has been covered previously in PAM. This article will hopefully provide some insight into the mysteries of substrate heating and why we think it is one of the easiest ways to ensure your planted aquarium provides a long-term healthy environment for your plants and fish.

After you read the section describing the six factors we think are critical to successfully growing plants, think about how your own aquarium techniques might provide these factors. You may find that you are meeting most of the requirements already. If you feel that you are not getting the best results from your efforts, you might want to consider trying substrate heating in your next aquarium.

By making this investment, you will find that plants will grow healthy and will stay that way for many years. You will discover that there is much less problem algae. In fact, you may wonder if there is any algae at all! And finally, you can achieve these results without constantly adding supplements to the substrate to compensate for the lack of critical nutrients.

We will use the term "substrate heating" in a very generic sense. We define substrate heating as the use of a device that can create areas of relatively concentrated heat in the aquarium substrate. The concept of concentrated heat is key – to produce the claimed benefits, the substrate heater needs to generate convection currents in the substrate. A device that simply warms the substrate evenly will not produce all the desired effects.

To visualize how substrate heating works, think of a home fireplace. The fire creates an area of hot air. The hot air expands and rises up the chimney. To replace the rising hot air, cooler room air is drawn into the fireplace. This movement of air, known as the draft, is more generally called a convection current.

In an aquarium, substrate heating is claimed to produce the same effect. A typical substrate heater in the form of an electrical heating cable is like a long thin fireplace – it is designed to be laid in a serpentine pattern near the bottom of the substrate creating a linear "hot spot" (Figure 1). Water in the substrate around the cable is heated and expands. The heated water rises through the cooler substrate until it can mix with the free aquarium water (the water column). At the same time, cooler water from the water column can enter the substrate between the cables to replace the heated water. This sets up water circulation in the substrate and produces the benefits described in the next section.

You might think that any difference in temperature would create a convection current. This is mostly true in air but a substrate is a far more complex medium, more akin to a solid than to a fluid. There are three methods of heat transfer – conduction, convection and turbulent flow. Conduction is seen, for example, when you place your hand on a window in winter. The glass feels cold because heat is moving from your hand to the outdoors through the glass. Obviously, there is no movement in the glass (other than on an atomic scale, of course). At the opposite end of the spectrum is boiling water – an example of turbulent flow and something you wouldn’t want to create in your aquarium!

Thermodynamics theory shows that with low heat density, heat is carried through the heated medium by conduction and no material flow is present. At a certain heat density threshold dependent on the medium, convection currents will form in the medium. Above an even higher threshold, "boiling" will occur, producing turbulent flow.

So, the burning question is "What is the right heat density?"

Since aquarium substrates can be very complex (a mixture of water, fine gravel, sand, soil, laterite, fertilizer sticks, clay balls, kitty litter, peat moss and whatever else is on the current "hot setup" list), the calculations required to determine an optimum heat density are impossible to carry out. Unfortunately, due to the same factors, it is very difficult for the hobbyist to experimentally determine whether a commercial substrate heater or a do-it-yourself contraption is producing convection currents. So we must trust the manufacturers of the cables or apply their heat densities to our DIY efforts – more on that later.

Dupla, in their "10 Golden Rules for the Optimum Aquarium" says this: "the heating cable (on the bed of the aquarium) integrates the entire bed soil into the chemical and physical cycle of the aquarium by means of the upward current of water caused by it." They liken this to natural streams that are fed by ground water welling up through the substrate. The ground water brings nutrients with it, feeding the plant roots, and carries away plant waste material produced by the roots. But it is important to note that when we use substrate heating, we are not trying to copy nature. We are just trying to achieve the same effects in the substrate.

In our opinion, based on our 14 years of experience maintaining planted aquariums with and without substrate heating, the overall benefit of substrate heating is to provide long-term stability in planted tanks. If you tend to tear down your tanks every year or so for whatever reason, don't bother with substrate heating. If you set up a tank for the long haul (longer than one year), substrate heating can greatly improve your chances of success. However, based on our experience, substrate heating will not improve the rate at which plants grow. If you are looking for a Silver Bullet to improve plant growth, don't look to substrate heating. You might want to investigate CO2 injection instead.

The pros and cons of different planted tank substrates have been debated on the Internet for years, with little resolution of what constitutes the "perfect" medium (there are obviously many ways to grow plants successfully). Based on these discussions and on our experiences, we have developed our own list of important substrate factors. In no particular order of importance, we feel an optimum substrate should have the means to:

1) Provide warmth in the substrate for certain plant species (Barclaya longifolia, for example). In this case the substrate should be warmer than the water.

2) Provide warmth in the substrate to speed up biochemical processes. Reduction reactions, those which make nutrients more soluble and thereby more available to plant uptake, will take place at an accelerated rate (2-3 times faster for each 10 degrees C rise in temperature).

3) Transport nutrients from the water column into the substrate. Important nutrients are ammonium (primarily from fish waste), iron (from trace element additions), calcium, potassium and other trace elements. This will replenish substrate nutrients used by the roots and provide long term viability.

4) Transport harmful substances out of the substrate. Decomposition byproducts may be harmful to plant roots. There is also conjecture that plants give off low level toxins (alleochemicals) to keep other plants out of their territory. Successful species of weeds have developed this into an art form. If these toxins build up due to poor circulation, the plant may harm itself and its neighbors.

5) Provide a chelating medium that binds the reduced state of trace elements with an organic molecule, enabling the trace element to be adsorbed by root hairs. It is also conjectured that iron in the substrate (from iron-bearing materials such as laterite) can sequester phosphate, if the phosphate is able to move into the substrate.

6) Provide a reducing rather than oxidizing environment so that trace elements are kept in their reduced state (usable by plants) or are reduced from their oxidized state. Iron especially will rapidly oxidize in water with normal levels of oxygen.

In a Dupla "Optimum Aquarium", a plain gravel substrate with laterite clay in the lower 1/3 and heating cables at the bottom satisfies all six of these factors. The heating cables provide items 1 and 2 directly. The convection currents generated by the "spot" heat source of the cables provide for 3 and 4. The laterite in the bottom 1/3 of the substrate provides for 5. The slow convection currents, coupled with nitrifying bacteria in the gravel will reduce the concentration of oxygen getting to the bottom layer of the gravel, providing 6.

Alternatives to substrate heating can provide some but not all of these factors. A heating pad under the tank will tend to warm the entire bottom layer uniformly. This will provide 1 and 2. We believe the lower heat density the pad provides will go through the gravel as conduction and will not generate the convection currents that provide items 3, 4 and 6 and, thus, defeats the operation of 5.

Another popular alternative is a reverse flow undergravel filter (RUGF). This will provide for 2 and 4 and perhaps 6. It may provide for 1 if you heat the water before putting it through the RUGF. Item 3 is problematical since the water is usually filtered before going to the RUGF (to avoid injecting what is scientifically know as "crud" into the gravel) and trace elements will probably be oxidized in the filter (oxidizing is a biological filter's purpose). Item 5 is a problem because a RUGF will probably push the chelating material up and out of the gravel. A bigger problem with an RUGF is its inability to provide an even flow across the entire substrate. The flow should be very slow and it is likely that channels will form, providing a path of least resistance for most of the flow. Don't get me wrong, a RUGF may provide the 6 factors, but it would be difficult to get it set up with the right flow rate, an even flow across the substrate and proper mechanical filtering.

Contrary to popular belief, an old-fashioned UGF may be a viable alternative to substrate heating. We had amazing success with an 85-gallon tank filtered by an UGF run by small powerheads. An UGF will provide 2, 3 and 4. Item 1 would be very tricky to achieve, if not impossible. Detritus pulled into the gravel can provide 5 but 6 is almost impossible unless a very slow flow is used and that would be hard to do evenly across the whole substrate. After all, the main purpose of an UGF is to create an oxidizing environment to promote nitrification.

Other alternatives have been suggested but in our opinion they will probably not produce convection currents. A very common idea is to use easily available (and cheap!) reptile heating pads or blocks to warm the substrate. Obviously, these devices do not meet the "concentrated heat source" requirement. Some warming of the gravel is beneficial but the full benefits aren’t seen without convection currents.

Another idea discussed often on the Internet is to put the fluorescent light ballast under the aquarium, allowing the heat generated by them to warm the bottom glass. Again, this will warm the substrate but is not concentrated enough to produce convection. An old-fashioned tar ballast certainly gets hot enough but if it was placed against the glass the bottom would probably crack due to the stress with disastrous consequences.

Another idea is to use cables designed to thaw rain gutters in the winter. These could be used if they get hot enough. A critical safety issue is the voltage at which they operate. If they plug directly into the house mains, they could cause serious injury or death if the insulation was breached. Be very, very careful with water and electricity.

Various manufactures offer electric substrate heating systems. As far as we know, the only source for electric substrate heating cables in the United States is Monolith Marine Monsters (http://www.marine-monsters.com/). Their Web site offers 100, 200 and 300 watt cables along with electronic controllers.

Dupla and Dennerle are two European companies that we know of but neither is exporting their products to the United States. If you live in a country that offers these products, you are in luck. Dupla products were offered in the U.S. at one time and some sources might still have the cables. Another U.S. source was Sandpoint. They are out of business now but you may find some of their heating cables at swap meets or on-line auctions.

A company in Great Britain offers a heated water system that consists of a flexible hose, a pump and a heater. One end of the hose is wrapped around the heater and the rest is laid in the substrate. The pump moves water through the hose where it is warmed by the heater. The warmed water them moves through the substrate, giving up its heat to the gravel. The outlet of the hose is above the gravel where the cooled water exits.

Two successful DIY schemes have been used in lieu of commercial systems. One style mimics the electric cables offered commercially and the other uses heated water in a closed system. The electric system uses fine Teflon insulated wire-wrap wire as the cable along with appropriate transformers and controllers. The heated water system uses a PCV pipe manifold under the gravel and a plenum with a pump and heater. The system is closed to aquarium so you don’t need to worry about overheating the tank water and filtering the water before it goes through the manifold. Both types of systems are described in excellent articles archived in The Krib web site (http://www.TheKrib.com/).

More detailed information on substrate heating can also be found on our Aquatic Concepts Web site, (http://www.frii.com/~booth/AquaticConcepts/).

We will assume you are installing an electric substrate heating cable since those are the most common. There are four steps: install the cable, add the gravel, connect the power source and controller and set the temperature. If you are setting up a brand new tank and aren’t in a rush to get the fish and plants in, you might want to do steps 1, 3 and 4 first in a bare tank to make sure the temperature controller works properly.

Select a cable length that will be able to warm most of the substrate. Figure how much length it takes to run back and forth across the tank at the suggested spacing for your cables, typically about 2" apart. If an exact length is not available, you can zigzag the cables or space them closer to use up more length or not go to the very ends or back of the tank if the cable is too short.

Attach the cables to the tank before you add any gravel (Figure 1). Most cables come with some sort of anchor to hold them in place until the gravel is added. Dupla anchors also determine the spacing between cables. If the anchors use suction cups, wet the bottom of the tank to help hold them tight. If you are a perfectionist, now is the time to get everything just right. You might want to take photos at this time – it’s the last time you will ever see your cables until you tear down the tank!

After washing your gravel, put in a layer about ½" deep (don’t mix laterite with this layer – that makes things too messy). Carefully move some gravel under the cables so they are supported by the gravel and are not touching the bottom (Figure 2). If the cable touches the bottom of a glass tank, it will spend more time warming the glass than the gravel. This won’t cause any damage but will reduce the effectiveness of the cable. Having some gravel under the cable also help the convection currents just like the grate in your fireplace holds the logs off the fireplace floor.

After the clean gravel and cable are arranged neatly, mix laterite (or whatever you prefer) into 1/3 of the remaining gravel. Carefully add this on top of the cable, being careful to not disturb your earlier work (Figure 3). Hint: If you add laterite, wear rubber gloves when you handle the mixture. Finally, add the remaining clean gravel.

Most cables have a step-down transformer to reduce the mains voltage from 110 volts (220 volts Europe) to a safe 24 volts (or 42 volts for Europe). The transformer also isolates the mains power lines from the cable, providing extra safety in case the insulation should somehow be damaged.

The main side of the transformer is usually connected to an electronic temperature controller. The controller will cycle the cable power on and off to maintain a specific water temperature. Note that the water temperature determines the cable on/off cycle, not the actual substrate temperature.

A high-end system will have a microprocessor controller that can "learn" the right on/off timing for your tank setup. The gravel acts as a very large temperature buffer – it takes a long time for the gravel to heat up once the cables come on. Likewise, the gravel will stay warm for quite a while after the cable is turned off. This may lead to the water column getting cooler or warmer than it should before or after the cables are on. The microprocessor can learn the "lag" time of your setup and anticipate when to turn the cable on and off to maintain the optimum temperature.

An alternative to an electronic controller is a simple 24-hour light timer. It can be set to turn the cable on and off hourly or whatever period will produce the correct overall water temperature. Obviously, it will take some trial and error to find the timer period that is right for your tank.

The controller should be set to the temperature you select for your fish and plants. It is wise to carefully monitor the temperature for the first week to make sure the cables can provide enough heat and the controller is accurate. The lag created by the gravel mass may require you to set a higher or lower temperature at the controller to ensure a good average water temperature.

In some cases, the heating cables may not have enough wattage to maintain the tank temperature, especially in cooler climates in the winter. If you find you need a supplemental heater, look for one with a very precise temperature control. Set the heater thermostat so that it comes on 2 or 3 degrees F cooler than the desired water temperature. This ensures that the cable is on for the maximum length of time – the supplemental heater will only engage if the cable cannot maintain the desired water temperature.

What about using a heating cable in a warmer climate, especially if your house is not air- conditioned? Obviously, if the house temperature is above the desired water temperature the cables will not be turned on. If you really want substrate heating, you may find it necessary to use a chiller to keep the temperature down, an expensive proposition. If you are in this situation, you may be better off trying one of the alternative substrate circulation methods mentioned.

The authors are not aware of any information supported by scientific experiments that demonstrates whether or not substrate heating is effective. Various manufacturers such as Dupla, Dennerle and Sandpoint have offered commercial systems but none seem to agree on the specific details of the system. The avid Do-It-Yourselfer has only anecdotal evidence to draw upon when deciding how to construct their own system. We have used substrate heating for over 10 years and have collected informal experimental evidence that convinced us that substrate heating is effective if used properly.

In the past 14 years, we have had a multitude of large tanks set up with and without substrate heating. At one point in time, we had a 100 gallon tank set up as an "almost" Optimum Aquarium (only a heating cable missing) and a 90 gallon tank using the same setup plus a heating cable. At the same time, we had an 85-gallon tank set up with an UGF.

The 100-gallon tank without a heating cable did great for 18 months, supporting lush plant growth and mostly invisible algae. After 18 months, it started to noticeably degrade – slower plant growth and more and more problem algae. We spent another year trying various remedies but nothing turned the tank around in the long term.

On the other hand, the 90-gallon tank with a 250-watt heating cable continued to thrive. There was no visible algae and no special maintenance was required. This tank ran perfectly for 5 years before remodeling the house forced us to tear it down. The set up was reestablished in 1995 as a 120 gallon tank and has continued to perform beautifully to this day.

The 85-gallon tank with the UGF grew plants equally well but it was not as stable. We think it was sensitive to detritus building up in the gravel; a thorough vacuuming every 6-9 months would perk it up. The tanks with cables required no gravel vacuuming (except to remove light surface detritus occasionally).

Furthermore, the old 100-gallon tank was torn down and reestablished with a 100 watt heating cable. We now consider this to be "low heat density". This was a discus tank with an 84 F water temperature. Again we saw good performance for a period of time but then algae and growth problems developed. With the 1995 remodeling, this tank was torn down again and redone with a 200 watt heating cable. With the higher heat density, it has also been stable and thriving for the last 5 years.

Finally, in 1995, the 85-gallon UGF tank was replaced with a 100-gallon tank (running at 76 F) set up with a 150 watt heating cable. This tank also continues to perform flawlessly five years later.

From this evidence, we think the key benefit of substrate heating is long-term stability. Plants will grow fine without them if you can accomplish most of the six things we mentioned. Just pulling up plants for trimming every month will accomplish a lot (stirring up the gravel, moving roots out of their own alleochemicals, etc.). But the ability of substrate heating with the proper heat density to create water circulation in the substrate avoids problems of a stagnant and nutrient depleted substrate. Substrate heating provides a continuous source of fresh nutrients and a mechanism to remove waste products, keeping the substrate fresh and viable for a very long time.

When you starting planning your next Dream Tank, consider adding a substrate heating system. You may find that it is not that expensive relative to the total cost of everything else that goes into a complete aquarium system. You will find that plants will grow healthy and will stay that way for many years, there will be much less problem algae and you won’t be constantly adding supplements to the substrate to compensate for the lack of critical nutrients. Remember, unlike fish, plants can’t chase after food so YOU must provide the means to move the food to the plants.