Temperature Control

How to Manage Your Tank Temperature

  • A good range is 76° to 80°F (25° to 27°C). A few species need to be kept several degrees warmer, and some species require temperatures a few degrees cooler.
  • A thermometer is vital. A stick-on type enables you to check the temperature whenever you look at the aquarium.
  • For tropical fishes a reliable heater is necessary, at least during the colder months of the year. Many aquarists in temperate climates face the opposite problem during the dog days of summer—keeping the aquarium from overheating.

Advice for Maintaining Aquarium Heaters

  • Both submersible and hang-on heaters are available. All are thermostatically controlled, and many can be set for specific temperatures. Hang-on models are less expensive, since submersibles must obviously be water-tight. Most common are heaters in glass tubes, but titanium and stainless steel heaters are gaining in popularity due to their unbreakable design.
  • Some heaters have simple warmer-colder adjustments, and you must adjust them until your aquarium is stable at the desired temperature. Others have an adjustment designated in degrees to set a precise temperature without trial-and-error adjustments, and still others have one or more preset temperatures you can choose.
  • For normal room temperatures, figure on 100 watts for a 20-gallon, 175 watts for a 55-gallon, and 300 watts for a 100-gallon. If the room gets quite cold, use higher wattages. The water temperature should be stable.
  • Notice that the smaller the tank, the more watts per gallon are needed. For small tanks, 5 watts per gallon are recommended, while larger tanks can get by with 3 watts per gallon. Extremely large tanks may need even less than that. This is because small bodies of water lose heat much more quickly than large ones. Think of how much more quickly a cup of coffee will cool off than a hot bath. A 55-gallon tank will lose heat about half as fast as a 10-gallon tank.
  • You can use two heaters that total the needed capacity. If one sticks in the on position, the water will not overheat as quickly, and you have a better chance of saving your fish. If one heater dies, the other will be able to partially maintain the temperature, again giving you more time to discover the problem.
  • If your setup has a sump, that is a great place to put the heater, and if you have a plumbing circuit you can use an in-line heating module.

What to do When Your Tank Overheats

  • In hot weather, the first thing to do is to increase water movement. The warmer water is, the less oxygen will be dissolved in it, but at higher temperatures, your fish’s metabolism will be higher, increasing their need for oxygen. Moving water picks up more oxygen, and it also evaporates more, which cools the water. If ambient conditions bring aquarium water into the high 80s for prolonged periods, you should take steps to cool the water, and the simplest way is to replace the normal aquarium top with screening and position a fan to blow across the water surface. This greatly increases evaporation. You will have to top off the tank frequently to replace the evaporated water. You can use slightly cooler water than what is in the tank, but only a few degrees, or you may shock the fish.
  • A further step would be to freeze water in plastic bottles and place one at a time into the tank or in the filter. Make sure the bottles are small enough that the water does not cool down too quickly or too much—extreme changes in temperature are more harmful to your fish than constant high temperatures.
  • If your home remains very hot for long periods of time, you will need a chiller for your aquarium. Some are drop-in models that hang on the tank rim and put the cooling coils directly in the aquarium. Others operate completely outside the tank, and the water must be pumped from the tank, through the chiller, and back to the tank.
  • A reef aquarium is the most heat sensitive. Because reef invertebrates require extremely stable conditions, they can succumb quickly in a prolonged heat wave. In fact, most tropical fish can easily handle very warm temperatures for a week or two, but marine invertebrates typically cannot. For this reason, chillers are most common in reef setups. Chillers also provide an opportunity to keep cool-water fishes and invertebrates, freshwater or marine. There are many fascinating species from temperate habitats that require cooler-than-room-temperature systems.

Ideal Coral Temperature

What is the best temperature for corals? Can they take temporary cooling or heating? Steve Smith (Ogden, Utah) 

Most experts today recommend 80°F as the optimum temperature for corals, although older sources unadvisedly recommended lower temperatures. Corals can often take cooler water better than water that is too warm. Lighting and pumps can add a lot of heat to a system.

One solution is to equip your tank with a chiller to counteract hot days and heat from lights and pumps. Chillers are expensive, but heat buildup can kill corals, so their worth is obvious. Careful selection and placement of lighting and pumps can help prevent problems.