Aquarium heaters are essential in keeping your fish healthy. For an equipment that is so vital in the overall health of your fish, there is no obvious way to tell if the aquarium heater is not working. There are no alarms or red flag signals to indicate that the aquarium heater is not working. Sometimes you can go on for weeks and not know that your aquarium heater is broken!
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How to Tell If Your Aquarium Heater Is Not Working?
There are 6 ways to check to see if your aquarium heater is not working.
1. Check the Aquarium Heater Indicator Light
Check your aquarium heater to see if there is an indicator light. Some aquarium heaters come with an indicator light that turns on when the heater is running. If your aquarium heater is working properly, you should see the light turn on and off throughout the day.
However, some aquarium heaters don’t come with an indicator light. If you have one of those heaters, then you have to resort to other methods on this list to tell if your aquarium heater is working or not.
There is also a chance that the indicator light turns on but it’s not heating up your aquarium correctly. You should always double check the water temperature to ensure that your aquarium heater is functioning properly.
2. Use a Thermometer to Check the Water Temperature
The easiest way to tell if your aquarium heater is working is by using an aquarium thermometer to check the water temperature. I use one of those floating thermometers with colored background. It’s easy to use because I can easily tell based on the colored background if I’m within the safe temperature range.
The only thing about the floating thermometers is you don’t get an accurate read. If you want an accurate temperature reading, you should get a digital thermometer.
The reading that you get from the thermometer should be able to give you an idea if your aquarium heater is working or not. If you are within the range of safe aquarium temperature of 76-80F (25-27 C), then your aquarium heater is most likely working.
However, I suggest you double check to make sure your aquarium heater is really working. Your water may just be warm because your room is warm. Double check that your aquarium heater is working by doing the other items on this list.
3. Use Your Sense of Touch to Check that the Aquarium Water is Warm
Stick your hand (or finger) in the aquarium water to see if it’s warm. You can usually tell if the water is too cold or too warm. If you have another aquarium at home, stick your hand in one and then compare it to the water temperature of the aquarium in question. This will give you an idea if something is wrong with your aquarium heater.
This method is not very accurate since it’s just relying on your sense of touch. You need to double check that your aquarium heater is working correctly by doing other items on this list.
4. The Aquarium Glass Feels Too Cold or Too Hot
Put your hand on the exterior aquarium glass. It should feel slightly warm. If it feels too cold or too hot, then something might be wrong with your Aquarium water heater.
This method is not very accurate since it’s just relying on your sense of touch. Therefore, you need to double check that your aquarium heater is working by doing other items on this list.
5. Are Your Fish Dying?
You will know something is wrong with your aquarium ecosystem if your fish are dying. If you are finding a dead fish every so often and don’t see any signs of disease, there is a chance the problem is a malfunctioning Aquarium water heater. You have to dig deeper and do some troubleshooting to find the root cause of the fatalities.
6. Did Your Fish Stop Breeding?
If your fish used to be prolific breeders and then all of a sudden they stopped breeding, then that could be attributed to a broken aquarium water heater. Most tropical fish prefer to breed when the water is warm. That is why breeders usually increase the water temperature to stimulate breeding.
If your fish stopped breeding all of a sudden, it could be because the water is too cold. You need to check the water temperature with a thermometer and double check that your water heater is functioning properly.
Use our article on How To Tell if Your Fish is Happy and Healthy (infograph) to check if your fish are thriving.
What Happens to Fish if the Aquarium Heater is Not Working?
If your Aquarium Water Heater is broken, your fish will start dying and they will no longer breed. Water that is too hot or too cold can be fatal for your fish. Water that is too cold will cause your fish to stop breeding.
My Personal Experience with a Broken Aquarium Heater
I recently had first hand experience troubleshooting a broken aquarium heater in my son’s 10 gallon tank. He had one of those water heaters that don’t have the indicator light. So there was no way for us to know that the aquarium heater was broken.
So how did I find out his aquarium heater was not working? It took me a few days to discover that his aquarium heater was broken.
First, I started to notice that quite a few of his fish were dying. They didn’t look sick but they just kept turning up dead every so often. I also started to notice that his guppies and swordtails have stopped reproducing. They used to be prolific breeders and then all of a sudden they stopped breeding. That was a red flag for me that something was off with the water temperature.
Then when I was cleaning my son’s aquarium and my daughter’s aquarium, I noticed there was a difference in water temperature. His aquarium water felt colder than the water in my daughter’s aquarium.
I started to investigate the water temperature in my son’s aquarium with a floating thermometer. His temperature was coming in too cold, below the safe aquarium temperature range of 76-80F (25-27 C). I knew right away his aquarium water heater was broken, I immediately went out and got a replacement water heater.
What to Do If Your Aquarium Water Heater Is Not Working?
Once you have confirmation that your aquarium water heater is not working. You need to get a replacement aquarium water heater immediately.
Types of Aquarium Water Heater
There are three main types of Aquarium water heater in the market. I have listed down the pros and cons of each type of Aquarium water heater.
1. Preset Aquarium Water Heater
The Preset Aquarium Water Heater is a water heater without the knobs/dials to adjust the temperature. The temperature is preset to an optimal temperature. Usually around 78F or 25.5C.
- Great for beginners
- No need to adjust the temperature
- No need to check the temperature to see if you have it adjusted to the correct temperature
- Not ideal for advanced fishkeepers looking to breed fish because you will want to keep the water warm
- Will not work for fishkeepers looking to keep fish who likes cooler water (Danios, Goldfish, White Cloud Minnows, etc.)
- Will not work for fishkeepers looking to keep fish who likes warmer water (Discus)
I personally like the Aqueon Preset Aquarium Water Heater. I like that it lights up when running so you can tell if it’s working. It is also shatter resistant which is great for my kids aquarium because I get nervous with the glass heaters. I also like that it is compact and it is set at 78F (25.5 C). With this heater, I don’t need to worry about it since there is no need adjust the temperature.
2. Aquarium Water Heater with Adjustable Knobs
Aquarium Water Heaters with Adjustable Knobs are glass heaters with knobs to adjust the temperature. Usually you can adjust up or down one degree with every click.
- Great for breeders who wants to control water temperature
- Great for fishkeepers with fish that needs cooler (goldfish) or warmer (discus) temperature
- You need to adjust the temperature
- You need to check the temperature to see if you have it adjusted to the correct temperature
- The knobs can feel tight and difficult to adjust underwater
- Temperature adjustments can sometimes be unprecise and inaccurate
I personally like the Marineland Precision Heater. I like that there is a LED light that turns on when it is heating the water so I can tell when my heater is working. I also like that the dials are easy to use. Plus the temperature display is really helpful because I don’t have to guess the water temperature.
3. Adjustable Aquarium Water Heater with Digital Display
Adjustable Aquarium Water Heater with Digital Display are water heaters with digital display so you can easily see the temperature. The adjustments are also more precise. Compared to the Aquarium Water heater with Adjustable Knobs, this is more precise and takes the guess work out of setting the temperature.
- Great for breeders who wants to control water temperature
- Great for fishkeepers with fish that needs cooler water (goldfish) or warmer water (discus)
- Precise temperature reading
- Easy to see temperature reading
- Different form factor (not your typical test tube shaped)
- You will need to adjust the temperature
I personally like the Freesea Aquarium Heater. I like the form factor and I like that it is compact. I also like that it is easy to set the temperature and pretty easy to read it too. There is no need to guess. I also like the LED light because it give me an idea that the heater is working.
How to Install Aquarium Water Heater
It’s important to properly install Aquarium Water Heater. Here is how to install aquarium water heater.
1. Attach the Aquarium Water Heater to the the Aquarium Glass
Typically, you can install the Aquarium Water Heater vertically. Some Aquarium water heaters can be installed horizontally. Follow the instructions of the specific water heater so you can attach it correctly in your aquarium.
2. Leave the Aquarium Water Heater Submerged Unplugged for at Least 30 Minutes or So
Don’t skip this step! Leave the aquarium water heater submerged in your aquarium water for at least 30 minutes. This will allow the heating elements to acclimatize to the water. Remember- Don’t Plug it In! Let it sit in the water for 30 minutes or more!
3. Plug in Your Water Heater
4. Adjust to Desired Water Temperature
If you have an adjustable Aquarium Water Heater, adjust it to the correct temperature for your fish. If you don’t have an adjustable Aquarium Water Heater then it should automatically run and get you to the preset temperature. Follow the specific water heater instructions when adjusting your aquarium water heater.
How to Prevent Water Heater Issues in The Future
There are a 3 things you can do to prevent future Aquarium Water Heater issues.
1. Change Your Aquarium Water Heater Every Year
Aquarium water heaters are essential to your fish well-being. Technically, Aquarium Water Heaters should last a few years but many fishkeepers replace their water heaters every year since they are fairly inexpensive.
2. Install Two Water Heaters in Your Aquarium
As insurance, you can install two water heaters in your aquarium. The second Aquarium Water Heater will ensure that you always have a functioning heater in case one breaks down. If you are planning to install two water heaters in your aquarium, you should split the total wattage. For example, if your 20 gallon tank needs 100 watt, then you should get two 50 watt Aquarium Water Heaters.
3. Have a Thermometer In Your Aquarium
You should always have a thermometer in your aquarium. Every day, when you feed your fish, take a quick look at the thermometer to make sure your water temperature is correct and that your aquarium water heater is functioning properly. Having a thermometer will let you know immediately if something is wrong with the water heater.
Other Aquarium Water Heater Issues
Aquarium Water Heater’s Indicator Light is On but Water Not Heating Up
Your Aquarium Water Heater’s indicator light is on but it’s not producing any heat. The water heater’s thermostat is broken since it’s not heating up the water. When that happens, you need to replace your Aquarium Water Heater as soon as possible. That’s why it’s so important to always have a thermometer in the aquarium so you can immediately gauge if your water heater is malfunctioning.
Aquarium Water Heater Keep Producing Heat Even at Lowest Setting
If your Aquarium Water Heater keeps producing heat even at the lowest setting, it’s possible your water heater’s thermostat is broken. It is not regulating the temperature correctly. When that happens, you need to replace your Aquarium Water Heater immediately. You don’t want your water to heat up too much to the point that it will cook your fish!
There is a Crack in the Aquarium Water Heater
Aquarium water heaters are sealed in a glass tube and that glass can crack. Usually, it cracks when the fishkeeper forgets to unplug the heater when cleaning the fish tank. The water heater continues to heat up and when it is submerged in newly refilled aquarium water, the glass cracks. When that happens, you need to replace the Aquarium Water Heater.
Be careful handling the aquarium water heater. Keep the water heater unplugged.
What Happens if I’m Using a Water Heater that is Too Powerful for my Aquarium?
If you are using an aquarium water heater that has a high wattage and is too powerful for your fish tank size, your water heater will still be able to regulate the water temperature. Your aquarium water heater will just turn on and off less often. It will also look awkward in your aquarium since a powerful water heater will be much larger than the less powerful versions.
What Happens if I’m Using a Water Heater that is Too Weak for my Aquarium?
If you are using an aquarium water heater that has a low wattage and is too weak for your fish tank size, it will still be able to regulate the water temperature. The aquarium water heater will just have to work harder to heat up the water. Overtime, this will overwork your heater and may cause it to break down.
What to Do if the Aquarium Water is too Hot?
Your aquarium water may become too hot when the water heater malfunctions or if it’s a hot summer day. In order to cool down the water, you can add a bag of ice cubes and let it float in the water. Once it cools down the water temperature, remove the bag of ice cubes.
Remember, it’s okay if water temperature gets up to 80F (27C), that is not a reason to cool down the water. You only resort to putting in the bag of ice cubes when the water temperature gets dangerously hot.
If the water heater is not working, you need a replacement immediately. If the warm water is a result of a hot room, you don’t need to replace the water heater since it’s working properly.
What to Do if the Aquarium Water is too Cold?
Your aquarium water may become too cold when the water heater malfunctions or when there is a power outage. In order to minimize heat loss, you can wrap a blanket around the aquarium glass. If the water is too cold, you can warm it up by floating a hot bottle of water (make sure it’s not a plastic bottle that will melt in the fish tank).
Remember, it’s okay if water temperature gets down to 60F range. You only resort to putting in the bottle of hot water when the water temperature gets dangerously cold.
If the water heater is not working, you need a replacement immediately. If the cold water is a result of a power outage, you don’t need to replace the water heater since it’s working properly.
Will My Fish Be Okay if I Lose Power?
If you have a power outage and it’s for a short period of time then you don’t have to do anything. Your fish will be fine. However, if the power outage is longer than 2 hours, you may want to wrap a blanket around your aquarium glass to minimize heat loss specially if it’s in the middle of winter. If it’s a warm summer day, you don’t need to wrap your aquarium with a blanket.
What Size Aquarium Water Heater Do I Need for My Fish Tank?
The size of the aquarium will determine the water heater wattage. Since a smaller aquarium will lose heat faster because there are more surface area, you will need proportionally higher wattage to keep the aquarium warm than compared to a larger fish tank. For example, a 20 gallon (75 liter) aquarium will need a 100 watt heater while a 100 gallon (380 liter) aquarium will need a 300 watt heater.
Below is a chart with the number of watt heater needed for your aquarium size.
|Number of Watts
|5 gallons (19 Liter)
|10 gallons (38 Liter)
|15 gallons (57 Liter)
|20 gallons (75 Liter)
|25 gallons (95 Liter)
|30 gallons (113 liter)
|40 gallons (151 Liter)
|50 gallons (189 Liter)
|55 gallons (208 Liter)
|65 gallons (246 Liter)
|75 gallons (284 Liter)
|90 gallons (340 Liter)
|125 gallons (473 Liter)