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Most common aquarium fish hail from tropical climates; they need water temperatures higher than room temperature. Even within their viable range, fish respond poorly to sudden changes in temperature, so you must carefully employ your equipment to ensure the temperature is stable.
The majority of aquarium fish need temperatures between 75 and 80 degrees Fahrenheit. This range covers the ideal temperature for both freshwater and marine fish from the tropics. However, exceptions exist. For example, goldfish need somewhat lower temperatures -- from 68 to 72 degrees Fahrenheit -- to be at their best. Also, temperate and cold-water marine aquariums have become more popular in recent years, which often feature fish and invertebrates that thrive around 60 degrees Fahrenheit. Always research your individual species for their ideal temperature. Above all, avoid sudden shifts in temperature. Fish have a harder time adapting to sudden changes than they do to wrong temperatures. Avoid letting the temperature deviate more than 2 degrees a day.
You have several options in aquarium heaters. Hanging models attach to the lip of the tank with their control element above the waterline. Submersible units are designed to function completely underwater and drop in the tank. More expensive heaters fit in external equipment that transfer heat to the tank via sump. Whatever style of heater you use, make sure you get one whose capacity exceeds the volume of water in your tank. Heater output is measured in watts. Generally, you want to get 3 to 5 watts of heater power per gallon of tank. Ideally, you want to achieve this with multiple heaters, which serves to maintain temperature in case one fails. This helps keep temperature constant while you replace or clean a heater. Never plug in a dry heater. Always read the manufacturer's instructions.
In most cases, people need to heat aquariums. For those specialized situations where an aquarium requires cooling, chillers can handle it. For example, some tropical reef setups need such powerful lighting that they require a chiller to offset the heat from the lights. Cold-water marine fish also need a chiller since they thrive below room temperatures. Chiller units typically cost hundreds of dollars, since they are specialized equipment. They often have titanium heat exchangers because they metal works best and cheaper metals would corrode in contact with saltwater, breaking the unit and dissolving toxic metal into the aquarium water.
Sometimes, summer heat can make a tank too warm. In this situation, you might not have a chiller unit ready and waiting. To cool down a tank, you can freeze sterile water bottles and dunk them in the tank. The exact size will vary based on your tank. However, even in small aquariums, multiple bottles -- ideally three -- work better than one due to a more favorable surface-area-to-volume ratio. Monitor the temperature and remove the bottles if the temperature starts dropping too quickly. Unfortunately, there are a lot of variables, so this is a somewhat trial-and-error method of keeping the temperature constant.
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