Fisk er kaldblodede dyr. I motsetning til pattedyr og fugler, regulerer de ikke kroppstemperaturen sin selv og er i stedet avhengige av temperaturen i miljøet deres. Mange fisk foretrekker vann som er varmere enn romtemperatur. Det er viktig, da, å varme opp vannet i fisktanken slik at det er komfortabelt for fiskene å leve i. Immersible Heaters Generally the least expensive type of heater, this device is hung on the side of the aquarium with the thermostat control above the water line and the heating element immersed in water. It is important to keep this part underwater at all times because it can overheat when partially uncovered. The immersion heater comes in glass and titanium varieties. Substrate Heaters These heaters are typically more expensive than the immersible type, but are easier to conceal, as they are underwater. Setting the temperature is also easier. Also called an undergravel cable heater, this heater consists of a thick flexible rubber cable that can be buried under the gravel. By heating the gravel (substrate), it heats the water uniformly and allows aquarium plants to take in more nutrients from the water. Filter Heater Filter heaters comprise a heating block that is designed to go in the filter. This block heats the water as it goes through the filter. Relatively new, these filters are more difficult to find and tend to be more expensive. Each is designed to fit a certain brand and model of filter. Heating Mats Rather than going in the water, heating mats heat aquarium water from the outside. These synthetic mats are placed beneath the aquarium and work similar to an electric blanket. Though generally not suited for large aquariums, it is hand for a series of small fish tanks that are not large enough to warrant the use of an immersion heater.