How do Aquarium Heaters Work? Everything You Need to Know

aquarium heaters

Starting a new aquarium is very exciting. My wife really enjoyed her first aquarium, but it didn’t do very well because she didn’t do her homework.

The Ultimate Guide To Saltwater Fish…

There are a lot of questions I started to ask that she didn’t have the answer to. When we bought our first aquarium heater, I was curious to know how exactly they worked…

An aquarium heater works by turning on when the water is too cold and shutting off when the maximum desired temperature of the water is reached.

Okay, there is more to it to that so let me explain the technical part in more detail and then give you some advice on the different types of aquarium heaters floating around out there.

When your aquarium heater is plugged in an electrical circuit is completed which sends heat out into your aquarium. So, what regulates the aquarium heater to keep it from heating the water too hot?

Your aquarium heater is designed with two bands of metal inside it that touch a contact point to complete the electrical circuit. When the bands of metal reach a certain temperature, caused from all of the surrounding water reaching the same temperature, they expand or move away from the contact point which breaks the electrical circuit.

When the circuit is broken, the temperature of the surrounding water falls again cooling the bands of metal which move back to the contact point, completing the circuit again and the water temperature raises again.

There is a knob or some type of control you can set that changes how high the temperature needs to be for the metal band to release from the contact point. This knob or control is called a thermostat. This is a constant cycle that keeps the water temperature within the proper range for the fish in your tank.

Now that we have the boring technical jargon out of the way, let’s talk about the different types of aquarium heaters and the principle behind how they work.

How Do Submersible Heaters Work–

This is the main type of aquarium heater that you will find in most fish tanks out there. They look like a long glass tube because the electrical heating components are wrapped around the ceramic that they heat, and all of this is encased in a glass tube. We don’t want the electricity getting into the water or your fish’s hair may stick straight up.

Submersible aquarium heaters get their name because they are completely submerged underwater in your aquarium. They usually have some type of knob attached to them so that you can adjust the temperature as discussed above. Some of the updated models have a temperature line, in addition to the control knob, that you can see through the glass.

Immersible or Non-Submersible Heaters –

These types of aquarium heaters hang at the top of your aquarium with the glass tube, basically the same as described above, hanging down into the water.

This is an older style of heater with the main advantage being that the temperature control knob is above the water line so you can adjust the water temperature setting without getting your hands wet.

However, these types of aquarium heaters are not as efficient as the submersible heaters because they or more limited in where you can place them because you have to hang them from the top of the tank.

We’ll get into aquarium heater placement in a bit though so let’s talk about in-line heaters.

Inline Aquarium Filter Heaters –

aquarium heater
ISTA In Line External Heater

These heaters are the favorite of some aquarists because they are built into the outflow line coming out of your filter. Your filter usually has a built-in pump that pumps water through it so the inline heater simply heats filtered water as it is flowing back into your tank. Voila! The returning water is already heated and filtered as it returns.

These heaters are becoming more popular among aquarists because:

  • They are less visible than submersible and immersible heaters.
  • It adds to the overall appearance of your tank.
  • The proper temperature water is distributed more evenly.
  • When the outflowing water is heated, it causes the heated water to spread more thoroughly throughout the tank.
  • When you are doing water changes or other maintenance you don’t have to unplug or remove the heater since it is built into the filter.

The only downside to an inline filter is the cost, they are more expensive. If the water isn’t flowing then the purpose of this heater is defeated. This is not likely to happen, but you should always have a backup heater in your aquarium anyways.

Pad or Bowl Heaters –

These are tiny heaters that can usually be set underneath a very small aquarium or inside it. They are mainly for a little single fish tank for say a betta fish. They are usually preset to a certain temperature, so you just plug them in.

Substrate or Under Gravel Heaters –

Also known as a cable heater, these heaters consist of a long flexible cable that you lay in a zig-zag pattern at the very bottom of your tank before you put your substrate in. These are the least used aquarium heaters, but some aquarium enthusiasts really like them for live plants.

These heaters create water movement among the substrate which can move more nutrient-rich water to the plant roots. However, this will only work with substrate materials that are larger than sand. Particles as small as sand will trap heat underneath because the sand becomes packed with no pores or space for the heat to pass.

Substrate heaters are not meant to heat your whole aquarium so you will need one of the above heaters and hopefully a backup one like we recommended above. Substrate heaters are precisely for plant health, but they should be included in any conversation about aquarium heaters.

Digital Heaters –

There are a lot of digital heaters now. This is simply any kind of aquarium heater that has a digital reading for the current temperature and the target temperature. Some of these are even of the submersible type where you can see the digital reading and control the temperature outside of the tank without getting your hands wet.

Proper Heater Placement –

The most agreed-upon way to place your submersible heater in your aquarium is to put it at an angle at the rear of the aquarium so that it heats the aquarium more evenly. If you can’t do this or if the only position you can place it is straight up and down, then you should put it in the flow of your aquarium water.

The reason to put your heater within the flow of your aquarium water is so the heat spreads evenly throughout your tank.

The flow of water is going to be coming out of your filter with its built-in pump or if you have a separate filter for a bubbler, it can be placed by that pump.

This will keep water at the proper temperature flowing around your tank easier. Improper heater placement can cause areas of stagnant water in your tank.

To explain this better, let’s pretend like you have a longer tank. If you put your heater at one end of the tank and there is not enough water flow, what often happens is just a certain area of water gets heated and the heater shuts off before the whole tank is heated properly.

Just make sure you place the heater where there is good water movement. If you have a tank with poor or no water movement then you my need more than one heater.

Choosing an Aquarium Heater –

When choosing an aquarium heater, a few factors will influence your final decision:

The size of your tank


The type of fish that you have

If you have really active fish that are mischievous, bigger fish or even a turtle, you will want to get an impact-resistant glass filter or make sure there is a heater guard on the outside of your filter. A heater guard is usually a plastic cover with vents. It protects the glass from impact while also letting the heat escape through the vents…

Always research the fish you are going to put in a tank and make sure they are compatible according to personality and the water temperature they usually live in because the water temperature is going to stay the same…

For example, tropical fish are so named because they live in warmer tropical waters, so you shouldn’t mix them with cold-water fish.You’re trying to mimic your fish’s natural environment that they lived in the wild, so do your homework and make sure you mix fish from the same type of environment that aren’t going to fight with each other.

Your budget –

If you have a low budget just make sure your heater is going to heat your whole aquarium properly. Just keep a close eye on the water temperature. You should at least try to find a cheap backup heater just in case.

This ensures that if one heater goes out at night there is still another heater heating your tank. You really should monitor the water temperature and heaters daily to make sure your heaters are working.

Your personal preference

Once you decide what size of heater you need, whether you need a guard, and your budget, the rest depends on your personal preference.

Some people don’t care as long as they have fish. Some people really get into the design and appearance of their tank. An example would be the inline heater we talked about above which is almost hidden from sight.

Most aquarists like to have the thermostat (heater control) outside the water so they don’t have to get their hands wet to adjust the temperature.

Don’t forget about substrate heaters –

Remember, if you have a substrate with live plants, a substrate heater can be very beneficial to your plant health. You can have a substrate heater along with two normal tank heaters because all of the heaters are adjustable to shut off at a certain temperature.

Temperature Controller: An Extra Layer of Safety for Your Aquarium Heater –

aquarium heaters
Inkbird ITC 308 Digital Temperature Controller

If you can afford to, buying a temperature controller is a good idea. You just place the temperature controller in your tank and set it to your desired temperature.

It plugs into your wall outlet and has a socket to plug your heater into. When your aquarium water reaches the desired temperature, the temp controller shuts your heater off. When the water temperature gets too low, the temp controller turns your heater back on.

The chances of your heater overheating are very low but if it does happen, the temp controller will shut your heater off and save your fish’s life. It is just a good precaution to have if you have the budget for it.

You Should Also Buy a Thermometer to Keep Track of Your Tank’s Temperature

Thermometers are on the cheaper side of aquarium equipment so you should be able to buy a couple regardless of your budget.

Stick-Em Thermometers

You stick these to the outside of the tank. They can be hard to read. Panel light comes on when temp changes

Hanger Thermometers

Hang from the top of the aquarium. They are a tube containing mercury that runs down into the tank. The mercury rises to the current water temp.

Digital Thermometers

Sticks to the inside of the tank with suction. Runs on Batteries. Gives a digital reading.

Wireless Thermometers

Sensors placed in the tank use radio waves to send the temperature back to the main unit.

Floating Thermometers

Floats around the aquarium and uses mercury to measure the temperature.

Important Tips to Remember When Dealing With a Heater –

The quality of your heater

Don’t sacrifice quality regardless of your budget when buying a heater. If you buy a quality heater the first time, it will save you money in the future from cheap heaters breaking down and having to be replaced.

You will also have to replace fish if they become sick and die from a faulty heating system. Your heater and filter are the most important part of an aquarium.

Is Your Dog More Important Than Your Fish?

If you had to leave for days at a time would you just leave your dog alone the whole time? I hope you said no because you should treat your fish the same way. Even if you buy a temperature controller, make sure someone you know can check on your floating pets also.

Don’t Turn Your Heater On or Off Right Away

When you first place your heater in your tank don’t turn it on right away or it may crack. You need to let it adjust to the temperature of the water for about thirty minutes. Likewise, when you take your heater out of the tank let it sit for thirty minutes after you turn it off before you take it out.

Remember to Get More Than One Heater

Even if you have a small tank you should have at least one backup heater in case your main heater fails. If you have a bigger tank you may have to buy more than one heater and make sure they are evenly spaced throughout the tank. Always have a backup.

Make Sure Your Heater Isn’t Touching the Glass

This is to make sure your fish don’t get shocked by the heater and make sure there are other places for your fish to hide other than between the heater and the glass.

Try to Replace Your Heaters Every 12 – 18 Month’s

This will help keep your heaters from going out. They aren’t going to last forever so you should try to beat the failure to the punch

Conclusion –

Having an aquarium is very exciting. It is fun to have when guests arrive to show and talk about. However, there is routine maintenance that is involved. It may seem difficult at first but once you get the hang of it, it is easy peasy. There is not much maintenance involved with a heater but monitoring your aquarium heater is very important. Just keep an eye on that water temperature, make sure all of your heaters are working every day, and change those heaters out every 12 – 18 months depending on the quality of the heater. Remember, your fish’s life depends on it!

More Frequently Asked Questions Related to How Aquarium Heaters Work –

How Long do Aquarium Heaters Last?

No one can really answer this question, but heaters do overheat over time. It is good practice to change them out every 12 – 18 months.

How Fast Do Aquarium Heaters Work?

It shouldn’t take more than 24 hours to heat your tank the first time. After that it takes about an hour to raise your tank by one degree Fahrenheit as long as you have the proper wattage for your heater/heaters which is 5 watts per gallon of water.

How do I know if my aquarium heater is working?

If it is a new heater, check to make sure the light is on. Remember it takes a long time for the initial first heating so sleep on it. Make sure the water temperature is slowly rising. If it’s an older tank the water temperature will keep dropping.

How do I turn my aquarium heater on?

You can adjust the temperature setting on it and then plug it in. There is either a knob attached to it or a separate switch that stays out of the water depending on the type of heater. Just remember to let it sit in the water for half an hour before you turn it on so that the glass can adjust.

Do aquarium heaters turn off automatically?

Yes. They have a thermostat which is like your heater in the house. You set the temperature you want and the heater automatically shuts off when the desired temperature is reached.

How long does it take an aquarium heater to heat water?

It shouldn’t take more than 24 hours to heat your tank the first time. After that it takes about an hour to raise your tank by one-degree Fahrenheit as long as you have the proper wattage for your heater/heaters which is 5 watts per gallon of water.

What temperature should my aquarium heater be set at?

Always do your homework and research the proper temperature for the type of fish that you have. Always have compatible fish from the same type of environment. Remember, an aquarium is trying to mimic their natural environment. For example, tropical fish come from tropical waters so they will need warmer water while a cold-water fish will need cooler water.

Do aquarium heaters use a lot of electricity?

They use five watts for every gallon. For example, a 10-gallon tank will use a 50 watt heater.

Should I turn my fish tank heater off at night?

No, it automatically shuts off when the temperature is at the preferred setting, so there is no need to shut it off at night.

Scroll to Top