Fish are extremely sensitive to water temperature, and can have physical problems or even perish when it gets too hot or too cold. Each species has its own range of tolerable temperature range, but in most cases if the water heats up much more than 80 degrees Fahrenheit, your fish are in danger.
As water temperature increases, the amount of available oxygen in the water decreases. At the same time, the warmer a fish becomes the more oxygen he requires. This means that fish have a difficult time taking in enough oxygen in water that is too warm and can asphyxiate, or suffocate, and die. A fish gulping for air at the surface may indicate that the water temperature is too high for him to take in enough oxygen. The exact temperature at which this occurs depends on the species of fish, with some able to tolerate much higher temperatures than others. Research your fish before you get them to ensure they're suited to your tank's temperature.
Aquarium water that is too warm can deteriorate quickly for several reasons. An initial increase in temperature can stimulate a fish's appetite, and the increased food intake results in additional waste. This waste causes nitrate levels to rise, which upsets the pH balance. In addition, warm water encourages rapid growth of bacteria and algae, further upsetting the pH balance. A drastic change in water quality stresses the fish, who may become ill as a result.
As long as the water temperature does not rise above tolerable levels, a slight increase in the amount of heat can cause many tropical fish to grow at a faster rate. This may seem like a positive effect if you desire large fish, but it comes at a high price to the fish's health. A fish that grows too fast may have physical problems and a shorter life span, as well as dulled color.
Hyperthermia is overheating. Most fish, including those who are popular aquarium pets, are poikilothermic, which means their body temperature is the same as the water. When the temperature rises above tolerable levels, the fish's internal temperature becomes dangerously high. If the water does not cool off, his organs begin to shut down and he will die. Rapid cooling of the water is not desirable, however, since fluctuations can be as hard on fish as water that is too hot.