Do You Need a Heater in a Freshwater Fish Tank?

Different types of fish have adapted to various temperature ranges. However, most common freshwater aquarium fish come from the tropics and prefer water slightly warmer than room temperature. Because of this, you'll need a heater for most freshwater aquariums.

  1. Tropical Fish

    • Tropical aquarium fish all prefer water between 74 and 80 degrees Fahrenheit. Some species prefer water a few degrees warmer or a few degrees colder. However, almost all freshwater aquarium fish will be comfortable in this temperature range. In most regions, you'll need to heat the water to keep them comfortable. Aquarium heaters both heat the water and help keep the temperature stable.

    Goldfish

    • Goldfish are a special case. Goldfish originate in subtropical and temperate regions. They prefer their water around room temperature or colder, specifically 68 to 72 degrees Fahrenheit. For this reason, goldfish do not require a heater in most situations. Since goldfish prefer colder water, they should not share an aquarium with tropical fish. The aquarium would either be too cold for the tropical fish or too warm for the goldfish. Incorrect water temperature can stress fish, so only keep fish with the same temperature preferences together.

    Picking a Heater

    • You should select a heater based on the size of your aquarium. Aquarium heaters are rated by their wattage. For larger aquariums, you want 3 watts of power per gallon. So a 40-gallon aquarium would need a 120-watt heater. For smaller tanks, you want 5 watts per gallon, because smaller aquariums lose heat more easily than large aquariums due to their greater surface-area-to-volume ratio. Ideally, you want to get more than one heater. This is insurance in case one heater fails, so you'll have time to replace it. For example, if you have a 10-gallon aquarium, you would want 50 watts of heater power, preferably in two 25-watt heaters.

    Guidelines

    • You want to keep the temperature in an aquarium stable. If your aquarium is too cold for your fish, change the temperature slowly. Sudden changes in aquarium temperature are more dangerous to fish than the wrong temperature, even when the change is toward their ideal environment. Avoid changing the temperature more than two degrees per day. Tropical fish come from areas where there is little seasonal variation in temperature.