Do tropical aquariums need a heater?

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AC Members
Jun 19, 2004
Austin, TX (Born NYC)
Title: Do tropical aquariums need a heater?
Author: Rohn DeSilva
Date: Dec 8th 2008

I have been asked numerous times if a heater is absolutely necessary for a tropical fish tank. My answer is no. But many experts advise on getting heater and setting the temperature at specific setting. According to these experts water temperature needs to be at a constant without fluctuation to prevent temperature shock. This argument is scientifically nonfactual. I will explain why.

I have a database consisting of more than 60,000 species of tropical fishes. From this database, the average minimum and maximum temperatures after normalizing the curve is 46°F and 101°F.

What does this mean? If you look at any profile of any type of fish, they list temperature range which the specific fish can survive. The temperature is usually listed as minimum and maximum. The reason for this lower and upper bound is to reflect the temperature conditions of their natural environment.

Guppies are one of the most common and widely kept aquarium fish. They are easy to keep and can survive under adverse conditions. I am going to use this species to illustrate my point. The most conservative temperature limits for guppies are 62°F and 86°F. The native environment for these fish is the swamps of South America where the water depth is a mere few meters. To my knowledge, these swamps are not installed with heaters kept at a constant temperature. Thus temperature of the swarm fluctuates between 62°F and 86°F. In the day time the water is heated and during the night the water temperature falls. These high and low temperatures are what is provided in most profiles. And in this case it is 62°F and 86°F.

Can this temperature fluctuation cause temperature shock? The simply answer is no. This fluctuation take place over 24 hour period at a slow rate. The water temperature doesn't jump from 62°F to 86°F or fall from 86°F to 62°F in few minutes. It happens gradually as the sun goes down and the air temperature cools down, and thus effecting the water temperature.

How does this effect your aquarium? Before I go on to explain this, I am going to give you a brief explanation of the thermodynamics of water. In nature, water temperature of a body of water is dictated either internally or externally. Internal causation is when the water is heated dgeothermally. While external causation is when the water temperature is effeced by the air temperature. We can rule out the first since guppies (and most tropical aquarium fishes) do not live in such conditions. Thus we can look at the effect of water temperature due to air temperature more closely since that is the main causation.

There was a temperature versus depth study done by UCLA. The temperature was measured by bathythermograph. The experiment was conducted when the air temperature was 18°C and thus the graph intersect the y-axis (or Temperature) at 18°C (64.4°F). Therefore at the surface water, temperature is identical to the air temperature (i.e depth of 0 meters).

The temperature versus depth graph is a linear till about 125m and then logarithmic further down. According to this data, temperature decrease of 7.8°C or 46.06°F occur at a depth of 125 meters. The temperature drop till this depth (125M) is has linear coefficient such that for every 1 meter depth, a temperature drop of 0.36848°F is observed. But the temperature drop beyond this point is dictated by complex exponential function.

This experiment was conducted for large body of water and the source of light as the sun. Compared to this, our aquarium are very small and thus have direct impact and higher correlation with the air or room temperature. And thus we can assume that water in an aquarium will be only slightly different from that of the air temperature or the room temperature. And since the water in our aquarium is effected by the external light from all four side, the temperature of the water remain stable and close to the surface temperature as possible. When I measured the temperature of water in aquariums with various height, I came to data closely resembling the UCLA project.

So if we are keeping guppies in a 55 gallon tank and the air temperature is 75°F, then the temperature drop at the bottom of the tank is only about 74.6°F. Since our guppies can tolerated temperature as low as 62°F, then our guppies will be perfectly fine provided that the room temperature doesn't drop below 62.4°F during the night. Conversely they will be perfectly fine, provided the room temperature doesn't rise more than 86.4°F. In most homes where majority of aquariums are, the room temperature only fluctuates between 85°F during the day and 75°F during the night in the USA. The constant temperature is maintained by us either using our AC or heaters. And thus if you have tropical fish that can survive in the temperature range between 74.6°F and 85.4°F, you do not need an aquarium heater.

Provided that the room temperature doesn't fall +0.4°F below the low limit and
rise above the -0.4°F the upper limit, you can keep a tropical aquarium without heater.
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