Aquarium Heaters, choosing the correct size/wattage.

March 19, 2018




Your aquarium is home to cold-blooded creatures who rely on the temperature of the water to sustain their body temperature. You will need to provide heat for your aquarium and maintain it at the right temperature for your fish.  A more detailed explanation can be found reading further.


There are many different heaters on the market today to choose from so rather than attempting to discuss with whom should you purchase your heater from and what name brand is the best, we will only start by discussing the distinct types of heaters there are to choose from:


Hang-on-Tank Heater: This type is only partially submerged and is less efficient but may provide basic heating.


Submersible Heater: You will get more consistent and efficient heating with a submersible heater.


Heating Cable System: This system is placed under the gravel or substrate and connected to a controlling unit. It can be useful for freshwater planted aquariums to eliminate dead spots. Because you would need to dig up your substrate if it needs to be repaired or replaced, it isn't good to use for saltwater reef systems.


Now that we can see the “type”, we can then discuss how to choose the correct size or wattage will work best for your application. 


Finding the Right Aquarium Heater Size


The basic rule of thumb for wattage is to use between 2.5 and 5 watts per gallon of actual water volume in the aquarium. You can use a single unit of the correct size or multiple units that add up to the needed wattage. But then you need to adjust for the temperature of the room and the temperature desired in the tank.


First, subtract the average temperature of the room the aquarium is in from the temperature at which you wish to maintain the aquarium water.  So as an example, the average temperature in my living room is 68 degrees Fahrenheit and my desired water temperature for my 75-gallon aquarium is 77 degrees Fahrenheit.  This gives me a solution of 9 degrees Fahrenheit, which when utilizing the Heater Size Guide below tells me that my aquarium will require a 250-watt heater to maintain my desired temperature.


When using the Aquarium Heater Size Guide, if the heating requirement is between levels, it would be best to move up to the next larger size and if utilizing two heaters, then they should be installed at opposite ends of the aquarium to heat it more evenly. In larger tanks, or a situation where the room temperature is significantly below the desired water temperature, two heaters may be required.




Factors affecting heater selections.


When choosing a hang-on and/or submersible heater, it is wise to use multiple units as it provides a more even distribution of heat and doesn't strain the heaters. If one goes out, the temperature may not plummet too dangerously until you can get a new unit. It's also smart to buy an extra heater to keep on hand as a backup.  Also, the heater tube length is important because heat rises therefore it should match the height of your aquarium.


Always check for heat sources and fluctuations as your tank may be located under a vent or next to intermittent heat sources that can make the temperature rise and fall and always be sure to unplug the heater when you are draining the tank. Otherwise, it may overheat when it is no longer submerged. 



Let’s Dive in Further…


Most aquarists understand that maintaining their corals within the proper temperature range is important to keep them healthy and growing. If the temperature is too low or too high, the zooxanthellae algae (see zooxanthellae algae), which most corals require for survival, will die or vacate the polyps. When the algae leave the coral, it exposes the white base calcium carbonate of the coral, it is sometimes called Coral Bleaching (see Coral Bleaching).


So, what is the proper temperature to keep your tank at?  Many prefer to keep their tanks between 75-78°F.  Many coral distributors recommend keeping your tank temperature at 82°F.


With so many recommended temperatures, what is the best temperature? Perhaps the best way to determine is to look at what the temperatures were when your corals were in the wild. If you look at Coral Reef Regions in the World (see Coral Reef Regions in the World), you will see where corals grow naturally. If you compare this to the indicated temperatures in NOAA's Sea Surface Temperatures (see NOAA’s Sea Surface Temperature), you will see that a clear majority of coral reefs are found where water temperatures are between 80°F and 89°F and into the lower 90's in the Red Sea.


Perhaps a more pertinent reef tank temperature question, particularly during the summer months, is "how high is too high"? When do you really need to crank up that electron sucking chiller you paid a half a month's wages for?


The first place to start is to determine where your corals originally came from.


The Indo-Pacific, Caribbean and the Red Sea are where most of the corals in the aquarium trade are collected. Perhaps the easiest way to find out is to ask the LFS or OLS where you purchased your corals where the corals were shipped from.


Since most aquarium corals are collected from the Indo-Pacific and the Caribbean where the water temperatures are routinely between 85 and 89 degrees Fahrenheit, perhaps you may not need to get too concerned if your tank temps do not rise above this level.


There are some potentially serious problems with higher water temperatures in saltwater aquariums, however. The higher water temperatures in saltwater (and freshwater, too), the less dissolved oxygen (DO) it will hold, which is detrimental to every one of the organisms in your tank. In general, dissolved oxygen levels are about 20% less in seawater than in freshwater. Without getting into the scientific calculations, formulas, and data, let's just accept that the saltwater at the equator, where the surface water temperatures are in the mid to upper 80's, holds about two-thirds of the DO as the water at the poles, where the water is cold.


Lots of people justify keeping their temperatures low by citing the difference in dissolved oxygen or respiration as temperature increases. When someone tells you that, it's obvious that they haven't crunched the numbers. The differences are extremely small. The difference in saturation between 76 and 86 is about 7%. Even considering the effect on respiration (again, assuming the Q10 model, which is an overestimate in most cases), in the event of an emergency, temps at the lower end grant you a few seconds to a few minutes of extra time before oxygen is limiting- not hours.

To give you an example of the impact temperature has on oxygen, imagine you had a 65-gallon tank that had nothing in it but a single, medium-sized clownfish which requires a Dissolved Oxygen level of about 7 mg/l while the Marlin requires a DO of about 3 mg/l.


If you consider only the direct impact of temperature on dissolved oxygen, that tank can support the clownfish for a bit over a minute and a half longer in the event of a power outage if it was at 78 than at 86. If you also assume that the clownfish follows the Q10 temperature dependence model (which my works suggests isn't the case) then you get about 5 extra minutes to solve the problem before the fish starts suffering from low oxygen. That's unrealistically low stocking though and doesn't include respiration from LR or LS. When you include more fish or LR and LS those times would be reduced dramatically.  Ironically, most of the critters in saltwater aquariums require higher levels of DO than do most of the other fish in the ocean. In the wild, most of the saltwater tropical fish are found on or near the reefs, as opposed to the deeper waters of the oceans, where the DO is higher due to the waves breaking on the reef, aerating the surface water.


That line of reasoning is probably the number one reason people run their tanks cooler than normal, but the presumption is wrong. Neither of these situations is worse than the other. The thermal stress threshold in reef animals is set by acclimatization, not genetics. The rule of thumb is that stress begins at 2-4 degrees above the average maximum seasonal temperature. Whether that seasonal maximum is 78 or 82 is immaterial. The margin of error is still the same. There are absolute genetic limits to what corals can be acclimatized to, but for most corals those aren't hit until temps in the 90s.


In addition, higher water temperatures also increase the rate at which material decomposes in saltwater. Bacteria increases its reproduction rate, which increases the consumption of O2, which lowers the DO level in the water.


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