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Related FAQs: Heating Freshwater Aquariums, Heater Impressions (Reviews) by Steven Pro, GFI Use on Aquariums, Electricity and Aquariums

Related Articles: Electricity and Aquariums GFI use

/The Conscientious Aquarist

Heating Freshwater Aquariums



By Bob Fenner


Heat and Heating: (pix: heaters, tanks with heaters, centralized systems, sumps, heat exchangers'¦)

Freshwater bodies, though being smaller than the world's seas, are more subject to change in temperature on the basis of their volume and exposure to outside influences. The smaller the aquarium, the more "show" shaped (tall and narrow), and its location near walls, doors and hallways, the greater the likelihood of thermal fluctuation. It's important to prevent very much or quick change in your systems temperature, as this can stress your livestock unduly.

<Or, maybe we should start with something like this?>

Specific heat is a scientific term describing the amount of energy necessary to raise (or resultant) from a given mass of matter changing a degree Celsius under specific conditions. Water is the standard here, with it taking (or giving off) more energy to change temperature than any other known material in the universe. Hence water does not change temperature readily'¦ and aquatic life is ill-suited to such rapid or great changes.

Prevention of Temperature Swings:

You can reduce the effects of thermal "leaking" by careful placement of your system. Putting the tank away from outside walls, drafty doorways and passages can add vastly to its heat content. If the tank must be located against an outside wall, do leave a few inches gap between the tank and wall itself.

Using a top to cover your aquarium can also be a surprising source of thermal saving. Evaporation can account for a few degrees difference in temperature. Keep the tank covered.

System Size:

Obviously the larger a system is, the more wattage it will require to keep thermally constant. Don't rely on any other source of calculation of actual gallonage than your own in determining the actual volume of your aquarium. For most (rectangular) shaped containers, a simple measure (in inches) of length, width and height, and multiplication of these values, dividing this sum by 231 will give you gallons. When in doubt, it's best to overestimate the size of your system, and provide more watts in resistant heating rather than less.

System Shape & Composition:

As with surface area of animals, the degree of exposure of "skin" of your aquarium presents more or less influence to chilling. More "cuboidal" shaped systems and ones made of thermally insulated materials (e.g. thicker glass, wood and fiberglass) are more temperature stable.

Heater Use

In almost all cases hobbyists should avail themselves of a modern thermostatic heater, even with cool and cold water systems like goldfish. By setting the thermostat near the lower limit you desire you will be assured that should there be a sudden drop in outside temperature, your systems occupants will be protected. A note here re the all-too common practice of unplugging heaters during warm months: Many folks do this thinking to save a bit on heater use and electrical consumption. There is much more to be lost by being caught by a sudden chill'¦ and the heater/s will not use appreciable power until actuated by the setting at low-temperature.

For smaller systems of a few to several gallons, 4-5 watts per gallon of a hang-in or submersible will do. Aquariums of several tens of gallons to hundreds do fine with 3-4 to 2-3 watts per gallon, depending on how well insulated these systems are, what the difference in water and air temperature is likely to be, how high you intend to keep the waters temperature'¦ A few examples: A ten gallon "mixed tropical" aquarium that will be maintained at the upper 70's F (up to perhaps the low 80's F.) is fine with a 50-75 watt heater. A Discus aquarium of a hundred gallons might best be served with two heaters of two hundred watts each, one placed at either end of the tank.

It's a good idea to "split up/divide" heater wattage into two units rather than rely on one larger unit in large systems. Think about this. One unit might fail for whatever reason, but the second functioning one keeps working. Utilizing dual units also ensures more even heating. If you're using submersible heaters, placing them down near the bottom, though above gravel, will also help to warm the system throughout, as warm water rises, as does heat in air.

Wherever they are situated, heater loss due to exposure to air while plugged in is a large source of loss of these aquarium tools. One way to prevent their accidental breakage is to wire them and water pumps through an extension cord that you can easily switch off during water changes. While doing routine water changes, turning off these pumps and your heaters, will prevent their exposure and overheating, and damage by rapid re-emersion in water.

Other Sources of Thermal Input:

Lighting and fluid-moving pump use (including powerheads and submersible pumps) can add a great deal of "waste" heat to a system, and these sources can become problematical in heat spells and hot summer days. They may become so important in overheating your tank that leaving lighting off or shifting it to nighttime use (instead of day) and leaving the top off the tank become necessary to maintain sufficiently low temperature.

Other Heating Means:

For folks with lots of space, a room or basement to dedicate, and/or many tanks to heat, the option of heating the airspace and insulating the room about the aquariums may be a better way of heating the aquarium water than individual heaters. Of course you'll have to bear the brunt of the tropical temperature and high humidity while enjoying your aquariums as well if using this technique.

Another possibility exists for multiple tank systems where central heating of the total gallonage can be done through a sump. Larger wattage heaters of a few thousand watts, heat-exchangers of differing types, and heat sinks can be employed to collectively mediate a large volume of water economically.

About Thermometers:

Aquarium thermometers are notoriously inaccurate, yielding misleadingly off information concerning the temperature of system water. Most, however are not as imprecise. That is, they consistently give inaccurate readings. For most systems it is fine to have any alcohol, dial, azeotropic mixture/digital type thermometer per system, to check when doing your feedings, checking on your livestock, and if you're interested, one "good thermometer" (possibly a drop-in mercury kind) to "check on the checkers" when and where real temperature accuracy is needed.


Even the smallest of environments that our livestock come from in the wild are consistent in temperature, and therefore thermal constancy is something we want to shoot for as aquarists. Careful placement, possible insulation and use of thermostatic heaters are the principal means to keeping our aquariums within preferred temperature range.

Re: Cheap heater mistake... Note re edit    1/23/07 Oh, and I found the caution about buying cheap heaters in the electrical article rather than the heater.  It might be helpful to add a caution to the heater article, too.  Just a thought. <<And a good one! Thanks.>> Celeste <<Tom>>

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