Last update: October 17, 1998
"Like I said, I just don't get the rationale [behind the coils]. I figured you would be the one to ask."
No problem. I think Dupla has been careful to hide the rationale to protect their product, i.e., keep it "magic". We didn't see the rationale at first either and set up a 100g tank without heating coils. Comparing the 100g tank and a 90g tank we set up with heating coils led us to hopefully understand more of what's going on.
I think a key concept is that we are NOT trying to mimic what happens in nature (even though the Dupla description implies that) but we are trying the achieve an equivalent biological effect. Let me answer a few of your points first, then comment on what we think is really going on.
1. The same temperature in the water and in the bed, just as in natural waters. (If the aquarium water is heated up e.g. by means of heating bars, only the water becomes tropically warm, while the bed soil remains at room temperature and the plants notoriously suffer from "cold feet").
"In an aquarium the water moves though the gravel fairly freely although slowly, at least as quickly as it would in nature, and maybe faster if you have an undergravel filter (UGF)."
First of all, without an UGF or reverse-flow UGF or the such, I don't think you'll find much water circulation in the gravel, especially if you have the smaller size gravel recommended for plants (2-3mm). What mechanism is there to cause movement? Water won't just move around by itself. Movement needs to powered by some source of energy.
In nature, you have sources of underground water moving to the surface or surface water moving to aquifers due to natural pressure differentials. Dupla mentions this in terms of "nutrient springs" in tropical streams. In our aquariums, there is no such natural pressures to cause any movement (except for UGF, etc.).
"If the water was kept at a constant temp, wouldn't the water, which surrounds the gravel particles, keep the gravel at that temp also?"
The water column will tend to keep the gravel at water temperature through conductive heating -- heat will "seep" downward. However, in glass tanks especially, the glass bottom is radiating heat into the room, cabinet, etc., unless insulation is provided. This will tend to keep the roots cooler than the water temperature. Even with insulation, I think you'll find the bottom of the substrate cooler than the top, just not as much.
"What's warming the substrate in the tropics? The warmth doesn't come from the earth, it comes from the sun. The sun warms the water, the substrate doesn't. The top of the water is always warmer than the bottom even in relatively shallow water."
Underground springs may bring up volcanic warmth (there are "hot springs" everywhere). Also, if the bottom is dark (old leaves, dark soil, whatever), it will pick up more heat from the sun than the relatively clear water would. I don't know how significant this would be (especially in a deep jungle) but in open streams it may be important.
2. Bed water currents which prevail in natural waters are copied; nutrients are fed to the roots, the materials discharged by the roots are carried away.
"OK, I understand that the warm water rises carrying with it the wastes (assuming roots discharge materials that need to by carried away) and is replaced by the cooler water carrying nutrients from above (oops, aren't you cooling the roots to the temp of the water above?). Is this copying nature? Warm water doesn't rise from the gravel, does it? What's down there warming it up? In nature the warm water is already on top being heated by the sun. The best you could hope for it seems is a turbulent stream in which the water is evenly mixed, but I don't think most of our plants come from conditions like that."
Actually, a lot of the plants do come from fast moving streams, especially plants from SE Asia like crypts.
Again, we're not trying to copy nature, just achieve the same effect. Here is a list of substrate processes I think are important (no particular order of importance implied):
A heating pad under the tank will tend to warm the entire bottom layer uniformly. This will provide 1 and 2 but I suspect the heat will go through the gravel as conduction and won't generate convention currents. Thermodynamics theory says that conduction will occur up to a certain heat threshold and then convection currents will be formed with more heat. I think the linear hot zones generated by proper spacing of the coils along with the higher temperatures of the coils will provide this. Yes, there will be hot and cool zones for the roots but I think the other factors outweigh this.
Schemes that use warm water flowing in tubes in the gravel won't work, in our opinion, because they can't generate enough concentrated heat. A typical system might wrap some tubing around a heater and pipe it through the gravel with a pump. The first foot or so of the tubing may get hot enough (though I doubt it) but the water will cool off rather quickly as it travels through the tube. If the tube is insulated enough to keep the water hot, then it won't transfer any heat to the gravel.
"If you truly need to keep you roots warm and want water flowing up through the gravel why don't you just surface skim your water and return it though a reverse flow UGF?"
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. 3 is kind of hard since the water is usually filtered before going to the RUGF (to avoid injecting crud into the gravel) and trace elements probably will be oxidized in the filter (oxidizing is a biological filter's purpose). 5 is a problem because a RUGF will probably push the laterite up and out of the gravel. Don't get me wrong, a RUGF may provide the 6 processes, but it would be difficult to get it set up with the right flows and even flow across the substrate and proper mechanical filtering, etc. A coil setup is a "no brainer" if you have the correct wattage.
UGF will provide 2, 3 and 4. 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.
We have three ~100g tanks with coils and one 85g tank with UGF. All grow plants equally well but the 85g is much more unstable. We think it is sensitive to too much detritus building up in the gravel; a thorough vacuuming every 6-9 months perks it up. The coil tanks require no gravel vacuuming and the 90g tank was rock solid biologically for at least three years. We replanted at that point because some of the plants had gotten out of control but we didn't "tear down" the tank - just replanted.
I think this is the key to the cables - long term stability. Plants will grow fine without them if you can accomplish most of the six things I mentioned. Just pulling up plants for trimming every month will accomplish as lot (stirring up the gravel, moving roots out of their toxin zone, etc.).
Unless you are really strapped for money, I would suggest buying the Dupla cables. They were designed for this purpose and are very rugged. Remember, once the tank is set up, the cables are the hardest thing to replace if they should go bad. If you need to redo the heating cables, you will need to tear the whole tank down and start over.
What cables should you buy? It depends on the size of your tank. The cables should be laid in a serpentine pattern across the bottom of your tank with the coils 1 3/4" apart. You can make a scale drawing to determine the length best suited to your aquarium. Here are the lengths of various Dupla cables:
Duplaflex 150 (50 watts) = ??? inches
Duplaflex 300 (100 watts) = 276 inches
Duplaflex 500 (150 watts) = 402 inches
Duplaflex 750 (200 watts) = 512 inches
Duplaflex 1000 (250 watts) = 669 inches
The Dupla cables come with cable anchors, saving you the trouble of figuring out how to keep them in place until the gravel is added.
The cables run with 24 volts AC so you need a step down transformer. To figure the amperage rating of the transformer you need, divide the wattage of the the cable by 24 and add about 10%. For example, the 150 watt cable needs a 6.875 amp transformer or larger. Large transformers may be found at electronic supply companies like Allied.
The coils will need some kind of controller to avoid overheating your tank. The best is an electronic temperature controller. These are available through various mail order suppliers. A low cost substitute is a simple light timer that has 24 hour programming. WalMart sells timers like this for around $15. Program a cycle such as "one hour on, two hours off" to control the water temperature. You will need to experiment to find the best cycle for your particular setup. Don't forget to compensate for the heating effects of your lights -- you may need to have the coils on longer at night than during the day.
The coils probably will not supply all the heat you need in the water. Be sure to use a normal aquarium heater along with the coils. If you use a temperature controller for the coils, set the auxiliary heater to 2 degrees below your desired water temperature. This way the coils will do most of the work -- if they can't maintain the temperature, the auxiliary heater will kick in.