Heaters

 (This article was originally published in February 1993.)

=========================================

Heaters

One of the basic elements of any aquarium setup to maintain tropical fish is a thermostatic heater.  When such a heater is working properly, it’s easy to forget that it’s even there.  This is a tribute to the reliability of modern heater designs, of course, and I for one appreciate their maintenance-free construction.  However, because they’re so easily forgotten, any problems that do occur can progress to the point where the health of your fish is endangered before you discover that something is wrong.  For this reason, it’s important to monitor the performance of your heaters and take proper care in their setup.

Although a number of designs are available, most of today’s aquarium heaters consist of a sealed glass tube which encloses heating elements and circuitry, including a self-contained thermostat.  There are two main types: hanging and submersible.  The former type hangs vertically from the top rim of the aquarium with the glass tube immersed in the water.  The latter type can be placed anywhere in the aquarium as long as most of the glass tube is below water level.  They are often attached to the aquarium glass with suction cups, and can be positioned vertically or horizontally, deep or shallow, depending on the particular setup.  Obviously, these are more versatile than the older, hanging style, but they are also more expensive.

The thermostat in an aquarium heater senses the water temperature and turns the heating element on when it drops below a certain preset level.  As the temperature rises above this level, the thermostat shuts the heater off.  The accuracy of this process varies among heater brands and models, but it is generally sufficient to maintain water temperature constant to within ±1°F, which is steady enough to cause no harm to fish.  Many brands advertise accuracy to within ±¼°F, which is far less fluctuation than most species experience in the wild!  Keep in mind that in nature, most fish do not live in a temperature-controlled environment.  Water temperature varies from sun to shade, deep water to shallow, day to night, and season to season.  It also varies with weather, often dropping when it rains.  As long as the temperature remains withing an appropriate range and any changes are gradual, small fluctuations (plus or minus a degree or so) will not affect healthy fish.

The most important factor to consider when selecting a heater is its power output.  If this is too low, it will not keep the tank warm enough, nor maintain a stable, uniform temperature.  If too high, it costs more than necessary, and any increases you make in the temperature will occur too rapidly.  In addition, if an overpowered heater sticks in the ON position, it can rapidly cook the fish.

Heater power is based on tank size.  A useful estimate of the power required in temperate climates is 3-5 watts per gallon of water to be heated.  I have found the lower end of this range to be perfectly adequate; however, in cooler climates (or if the tank temperature must be maintained more than about 10°F above room temperature), it would be desirable to provide 5 watts per gallon or even more.  As an example, a 50 gallon tropical tank kept at 78°F in a room kept at 68°F will require 150 watts of heater power; if located in a basement at 58°F, 250 watts or more will be needed.

Tubular heaters are available in a wide range of powers, from 25 watts right on up to 300 watts.  (For very large systems, 300 gallons or more, there are high-powered heaters available which are not located in the aquarium.  Rather, they are placed in-line with the filtration system and water is pumped through them, being heated as it goes.)  The higher-power tubular units are sufficient to heat tanks up to 100 gallons under normal condition, but I would strongly suggest the use of two half-powered units, especially in larger aquaria.  There are several reasons for this:

            1) Each unit acts as a “fail-safe” for the other.  In the event that one heater fails, any temperature swing that occurs will be smaller and the rate of change will be lower than if a single full-power unit had failed.  For example, if one unit won’t go on , the other will run and keep the temperature from dropping as far or as fast as if no heater remained.  On the other hand, if one sticks in the ON position, the other will run less, and the temperature will rise less (and more slowly) than if the sticking heater was full power.

            2) The two units can be placed far apart in the tank to provide a more even distribution of heat.  If one full-power heater, or both half-power units, are placed at one end of a long aquarium, the tank temperature will not be completely uniform unless there is a very strong water flow in the tank induced by the filtration system (more on this later.)

            3) For very large tanks (say, 125 gallons or more), only the in-line units are made with high enough power output that a single unit can do the job.  In such aquaria, you may utilize this type or multiple heaters of the standard type; if you choose the latter option it is important to select and set them up properly.

Whether you choose a hanging or a submersible type, there are several things you can do to maximize its performance and give your fish the most comfortable, thermally stable environment possible.  First among these is to obtain an accurate thermometer and check the temperature daily.  This is the best indication you will get of the proper operation of your heater, or of its malfunction if that should occur.

Another way to get the most out of your heater is to position it properly within the tank, and to select the unit, when purchasing it, with an eye towards proper positioning.  The heater heats the water that immediately surrounds it, i.e., that which is in direct contact with the glass tube. Water is not a good conductor of heat, and unless this warmed water is circulated throughout the tank, the water temperature in the aquarium will not be uniform.  In the absence of any water circulation, water heated by the heater will rise to the surface and form a layer.  The bottom of the tank will remain significantly cooler.  This stratification may be fine in lakes and oceans, but it is not desirable in an aquarium.

There are two ways to remedy this situation.  Since warm water rises off the heater, you can induce a circulation by locating the heating element as close to the bottom of the tank as possible.  As the heated water rises, cooler water sinks to the bottom to replace it, inducing a convection current in the aquarium.  By itself, this circulation will not be sufficient to keep all parts of the tank at a uniform temperature, but it helps.  It’s easy to place a submersible heater of any size hear the bottom, but how can you do this with a hanging model?  Several of these are available in different lengths — 8", 10", 12", 20", etc.  Select the length that will place the heating element (located near the bottom of the glass tube, farthest from the thermostat control knob) as close to the bottom of the tank as possible.

The other, more effective way to avoid stratification and “hot spots” in the aquarium is to locate the heater directly in the current provided by an airstone or power filter.  The water movement produced in this way thoroughly distributes heated water throughout the tank and ensures a more uniform temperature.  Alternatively, the heater (or heaters) can be located near the intake of a power filter.  Warm water that is drawn into the filter will quickly be spread to all parts of the aquarium.  However, because the current at a point near the intake of a filter is less than that directly in the outflow, this will be somewhat less effective.

Hanging heaters must be installed vertically, but this is not the ideal position (see diagram).  As warm water rises from the heating element, it moves along the tube of a vertically positioned heater and will affect the thermostat.  Proper water circulation will minimize this; however, if you have a submersible heater you can install it in a diagonal or horizontal position.  This may make it a little harder to reach the control knob, but once set, this should rarely have to be touched.

Many submersible heaters have a rubber cap covering the top of the heater and the control knob.  These are used to make the unit waterproof.  I find that they often make them adjustment-proof, as well.  It can be extremely difficult to turn a control knob that is covered with one of these caps, especially when it is wet.  I much prefer the brands that are waterproofed without these troublesome caps.

A number of current heater models have a temperature scale on them to aid in setting the thermostat.  My experience is that while these are useful guides, they are not absolutely accurate.  When installing a heater into a new (cold) tank, place it in the water in a convenient location (not necessarily its final position) where the thermostat control knob can easily be reached.  Be sure there is sufficient water circulation from a filter or airstone to thoroughly distribute the heated water throughout the tank.  Plug the heater in and look for the pilot light.  (If it’s not on, turn the knob until the light comes on.)  If the heater has a temperature scale, turn the dial to a point a few degrees below the desired temperature.  Now check the tank temperature every half hour or so.  When the pilot light begins to cycle on and off, the set temperature has been achieved.  Using the thermometer reading as your guide, make fine adjustments to the knob until the tank has stabilized at the desired temperature.  Be sure to give the tank time to equilibrate before making each adjustment.  (If the heater has no temperature scale, the initial setting above current tank temperature will be a guesstimate.  It’s better to err on the conservative side and have to increase the setting later than to set it too high and have to wait for the temperature to drop before resuming.)  Once this has been achieved, the heater may be moved to its permanent position.

It is not necessary to unplug your heaters during the summer months.  They may not go on very often (unless you have a bodacious air conditioner!), but they will place a floor on the aquarium temperature.  This is the exact reason they’re there, and there is no point in disturbing your careful equilibrium, or in removing your insurance protection.

Once a thermostatic heater is in place and properly functioning, it should be able to maintain a stable aquarium temperature for years without further attention.  This happy state of affairs is far easier to attain if the heater is selected and installed properly at the outset.