Your In-Depth Guide To Fish Tank Heaters

In Aquarium Heatersby PaulK


Aquarium heaters are one of the most unreliable pieces of equipment for your fish tank.

They are, however, necessary for the health and well being of your fish, so it’s important to find out more about them.

In this short guide you will learn:

  • Why you need a fish tank heater.
  • Types of aquarium heaters (and which is best).
  • What is the proper size heater for your fish tank?
  • Safety precautions (so you don’t fry your fish).
  • Where to place the heater for maximum effect.
  • How to know if it’s working.

Let’s get started….

Why Do You Need a Fish Tank Heater?

Fish are cold-blooded creatures and therefore unable to regulate their body temperature. Since they are not in their natural environment, we must do that function for them. Fish tank heaters are important to keep the temperature of the water warm enough for the particular species of fish you have in your aquarium. If the water gets too cold, your fish’s metabolism system slows down, affecting its immune system leaving the fish vulnerable to disease. You need to get a heater that has enough power or wattage for the size tank you have. A general rule of thumb is 5 watts per gallon of water.

Types of Aquarium Heaters

Hanging Aquarium Heaters: These are usually the cheapest kind of aquarium heater. They hang on the edge of your fish tank with a glass tube that goes in the water. Some brands have an adjustable heat setting and some brands don’t. The smaller ones will just heat the water to a certain temperature between 76 and 82 degrees. Hanging heaters come in many different sizes to accommodate smaller or larger tanks.

You have to make sure that these heaters are securely set on the tank otherwise if they fall off and the glass breaks you could electrocute the fish or start a fire.

Submersible Aquarium Heaters: These types of heaters can be fully submerged in the aquarium. They consist of a core heating element, such as a coil, with glass or ceramic casing to protect the heating element which is then enclosed by watertight plastic or glass or steel to make it submersible. They come with a clip for the suction cups that attach the heater to the tank. On most brands, there will be an adjustable temperature gauge at the top. Some brands have calibration for more accurate heating temperatures. They are a more efficient type of heater that offers better heat distribution.

Submersible heaters can be placed either vertical, horizontal, or at an angle. They tend to work better if you position them horizontally and low in the water column. This will allow the thermostat to get a more accurate reading of the water temperature.

It’s important to keep submersible heaters off of the gravel. The heat conductivity of the gravel might crack the glass heating element. You also want to leave a space between the heater and gravel, so fish don’t get stuck and get burned.

Aquarium Substrate Heaters: These are the least common type of heater. Substrate heaters are heating wires that are installed on the bottom of the tank before you add the substrate. The wire heats the gravel which then heats the water. They are better if you have live plants, so their roots stay warm.

In-line Aquarium Heaters: These type of heaters are attached to plumbing outside of the tank, usually associated with an external filtration system like a sump. A water pump is used to move the tank water through heater and filter and back into the tank.

In-filter Aquarium Heaters: The heater is located within the filter itself. Usually in canister filters, but sometimes in hang-on-the-back filters. The water flows through the filters and is heated at the same time. The water then returns to the tank all heated and filtered.

Which type of aquarium heater is best?

When it comes to aquarium heaters, quality is key. You don’t want to scrimp here. Preferably, you want a heater that is shatterproof, has an automatic shut off in case of malfunction, a controller so you don’t have to dip your hand into the water and separate digital thermometer to accurately, and easily monitor the water temperature. The reason you need a separate controller and digital thermometer is that all aquarium heaters can and do fail from time to time and you need a more reliable method to monitor the water temperature and to automatically shut off the heater in case of malfunction.

Now, you can buy these all separately or get this all in one titanium heater on Amazon. It’s shatterproof, reliable and has everything you need.

What Size Heater Should You Get?

If you get too low wattage heater for your tank size, the heater will be running constantly to try and keep up the temperature and will burn out faster. I recommend you get around 5 watts per gallon, so a 10-gallon tank would require a 50-watt heater. A 20-gallon tank would require a 100-watt heater or two 50 watt heaters. A 25 – 30-gallon tank would require a 150-watt heater or two 75 watt heaters etc. etc.

The advantage of using two heaters vs one heater is that you will get a more even distribution of temperature and you have a backup in case one burns out. On larger tanks over 75 gallons, you definitely want multiple heaters.

Get an external digital thermometer to monitor the water temperature. They’re more accurate than the cheap glass ones. Other things like having the tank too close to a window, or high room temperature could heat up the water fast, so keep an eye on the temp.

Most heaters will shut off automatically when the water reaches the desired temperature.

Where Does the Heater Go?

Submersible heaters are fully submersible in water. Hang up heaters hang on the edge and only the heating element goes in the water.

For a narrow, tall tank place the submersible heater towards the bottom horizontally not touching the gravel or other substrates. The warm water will rise to the surface and evenly heat the tank.

For a low, wide tank, if you only use one heater, put it at an angle in the center back of the tank.

If you use two heaters, put one vertically on each side in the back.

It’s important to put the heater in a spot where the fish won’t get stuck between the heater and something else like a decoration. If fish get trapped, they could get burned pretty bad. Make sure you have some hiding place for the fish, so they don’t feel they have to hide out by the heater.

Safety precautions

You’ll want to be careful, especially with cheaper glass heaters because they have been known to blow up and cause house fires. I have an article on the dangers of aquarium heaters here.

Here are some highlights.

  • Create a drip loop with the cord.
  • Use heaters that are shatterproof. Some fish can break the glass on cheaper models.
  • Use a GFCI (ground fault circuit interrupter) outlet. They are safer around water.
  • Attach the heater in the tank for 30 minutes before plugging in. Conversely, unplug the heater 30 minutes before you remove the heater.
  • Watch the heater setting vs. the temperature of the water. It will tell you if the heater is functioning properly.
  • Don’t overload the outlet with multiple plug-ins.

How do you know if your heater is working?

You need to have a decent thermometer in the tank to measure the water temperature. The temperature of the water should be the same as the heater setting when the heater is finished heating the water. If it’s not then the heater is not necessarily broken, it may need to be calibrated or the heater is too small for your size tank.

Heaters have indicator lights to tell you when the element is heating and when it’s done heating.

Various brands will have different indicators. Some will light red when heating and blue when done or red when heating and nothing when done. Read the owners’ manual to see what the indicators are. Some higher quality heaters, like the Eheim Jager, have a calibration dial that lets you calibrate the heater until the temperature of the water is the same as the heater setting.

You can run a test for the heater, by setting up two bottles. One with hot water and one with cold water. Put the heater in the hot water bottle. If the temperature of the water is hotter than the heater setting, then the indicator light should be off or whatever color is off. Next, put the heater in the cold water bottle. The heater indicator light should come on indicating that the heater is heating the water. Put the heater back in the hot water and the indicator light should go off again.

The ambient temperature of the room could be a factor as well. Tanks without heaters will take on the temperature of the room. Cold room, cold water in the tank. Hot room, hot water in the tank. Make sure your tank is not by a window with the sun beating down on it because that will overheat it too much. If the tank water gets too hot, the water cannot hold the oxygen that fish need to survive. Your fish could suffocate eventually.

Aquarium heater light on but not working.

Touch the heater to see if it gets hot. If it gets hot, then it may be heating the water slowly. Make sure you put the heater close to the water flow like a filter or air pump so the flow will distribute the heated water.

Do you need a heater?

Some breed of fish can survive without heated water, but it’s best to have a heater for most tropical fish. If the water gets too cold the fish’s metabolism starts to slow down, which affects the fish’s immune system and leaves the poor fish more vulnerable to disease.

Some freshwater fish don’t need a heater and can survive in colder temperatures. Here are some of the more popular breeds for cold water.

Goldfish: Very popular starter fish because they are tough and resilient. Don’t mix long body goldfish with round body fancy goldfish. The long body goldfish are faster swimmers and will nab most of the food.

Zebra Danios: Very active fish that like to be kept in large groups.

Guppies: Readily available in many attractive variations.

White Cloud Mountain Minnows: A very hardy fish that don’t require a lot of maintenance. Great for beginners.

Peppered Cory: Great bottom feeders. Fun to watch. Easy to care for.

Barbs: Easy to care for. There are gold barbs, cherry barbs, green barbs, and the two spotted barbs. A ratio of one male to two females is best.

Paradise Fish: Very colorful but they can get quite large. Up to 6 inches, so you need a larger tank. They are very territorial and will fight amongst themselves if you have too small a tank.

Bloodfin Tetra: Easy to care for and pretty hardy. They are active top dwellers and best kept in schools.

Cold blooded tropical fish need the heat!

Water temperature guide for Freshwater Tropical Fish

African cichlids: 76 F, 24 C

Betta: 76 – 82 F, 24 – 27 C

Discus: 79 – 88 F, 26 – 31 C

Neon Tetra: 75 – 79 F, 24 – 26 C

Angelfish: 74 – 78 F, 23 – 26 C

Oscars: 74 – 81 F, 23.5 – 27 C

Mollies: 72 – 84 F, 22 – 29 C

Platy: 68 – 79 F, 20 – 26 C

Loach: 75 – 81 F, 24 – 27 C

Pleco: 70 – 80 F, 21 – 26 C

5 Most Important Things to Remember About Aquarium Heaters:

  1. Get a reliable brand.
  2. Get a backup or use multiple heaters in case one fails
  3. Follow safety procedures. In this case, it’s important!
  4. Monitor the water temperature frequently.
  5. Get the right size heater for your tank.

When it comes to fishkeeping, what is your single biggest challenge or frustration? Leave a comment below.

This site is owned and operated by PSK Enterprises LLC. PSK Enterprises LLC is a participant in the Amazon Affiliate Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to

Spread the love