One of the most important factors to plan when putting fish in your aquarium, is the temperature of the water. And that is why it is so important where, in the home, you actually place the aquarium.
You want your fish to do more than simply survive in their aquarium environment. You want them to thrive. And, for this to happen, fish need for the temperature of the water to be stable. A constant shifting of the water's temperature from warm to cold and back to warm again weakens their immune system. And, a weakened immune system makes them more susceptible to illnesses.
The right water temperature is determined by the type of fish populating your tank. Tropical fish require warmer temperatures than do freshwater fish. But whatever species you have, you will have to monitor and control the water temperature.
In modern aquariums, keeping the temperature constant is a simple matter. It is very easily done by using a water heater which is controlled by a thermostat. All you have to do is to set the thermostat to the temperature you want and allow it to cycle the heater on and off as necessary. This way the water temperature, at least in large tanks, will remain consistent. However, just about all aquarium heaters are limited in the amount of heat that they can produce. That is why you have to be concerned about the room temperature as well.
So, for instance, if you determine that sixty eight degrees Fahrenheit is the optimum temperature for the fish in your aquarium, you should keep the room temperature that houses the aquarium as close to this temperature as possible. For example, if you live in a cold climate and allow your room temperatures to drop to sixty degrees or below when you are not at home, your aquarium heater may not have enough power to keep the water in the tank at the correct temperature.
What this means is that your room temperature and your tank heater are working hand in hand together to keep the water at a consistent temperature throughout the day and night. The temperature of the room should not be more than a few degrees different from the temperature of the tank.
Having water too warm is just as bad as having it too cold. Warm water cannot hold as much oxygen as cold water. When water in your tank is heated, it loses oxygen. This, of course, is bad for your fish. That's why as the water heats up, you will see them coming closer to the surface. They are trying to get access to more air. If the water is too warm, they won't die of heat, they will die of asphyxiation.
But, too much heat is also bad for fish as it can damage their fins as well as other parts of their body. Be aware, however, that heat can come from a variety of sources. It can come from the heat of the room, or it can come from the tank being positioned in direct sunlight. This is why the positioning of your tank is important - to avoid hot spots.