The name Tropicals, places too much emphasis on the idea of high temperature, for all exotic fishes. There is a number of them that do not come from the tropics, and a few of them that come from the tropics, do not come from particularly warm water.
It is understood that the majority of the exotic aquarium fishes, can not stand cold water, and rightfully so, because the fish can get sick easier, and on the other hand, many of them do not prosper in the higher temperatures, because they need more oxygen than the water can carry.
Placing them all in water around 80 degrees, is not the kindness that the Aquarius thinks, who likes high temperature. The one thing you should not believe, is that each specie has an exact degree of heat. Most of them have a tolerance of at least 10-degrees and can stand a 5-degree change over 8 hours, without injury.
As far as individual species is concerned, is to place delicate fishes in the warmest and most consistent places, and the hardier ones in the cooler spots. I believe a great deal of needless worry and expense is given to trying to keep aquariums or aquarium rooms within 2-degrees of a fixed point.
Nature does not supply such an environment, and it has been observed that fishes are stimulated by some change of temperature. The aquarists should not be too concerned about the variation between the heat at the top and the bottom of the aquarium. The variation in native waters is considerable, but the fish seems to handle it without trouble. They can choose what level in the water they like.
It all comes down to this: The average exotic aquarium fish is happy at a minimum temperature of 72-76 degrees. At times, you should never let the temperature drop down below 70 degrees, and if it does it would not hurt, but raise the temperature soon. Taking the temperature above 8o- degrees is alright, as long as theirs is enough oxygen in the water.
The breeding of most fish, means raising the water temperature above 80- degrees. If through uncontrollable causes the temperature drops down to low 60-degrees, or even 50-degrees, the thing to do is slowly raise it to about 80-degrees and keep it there for 24 hours or more.
The best way to keep a aquarium at the right temperature, is with a heater. Aeration equalizes top and bottom temperature. The most asked question is, "What size heater is required for a given gallon capacity?" This can not be answered intelligently without knowing the amount of cold to overcome. A general rule is 2 watts for raising each gallon 5-degrees above surrounding temperature. I favor conservative sized heating units, then if thermostat fails to function, the fishes will not fry.
It's alright for the fishes to swim from warmer to cooler water in your aquarium, but its entirely different changing them, from one tank to another, of which could vary in temperature of several degrees, either up or down. This is one thing that just must not be done. The effect may not be apparent at once, but it is seldom escaped. It usually brings about the "shakes" or "shimmies," Ichthyophthirius (ick), fungus or a general declination downhill. Changes should be made within 2-degrees of the temperature. You should check thermometers because they can be off as much as 4-degrees.
Source by Cathy Ludwig