Aquarium Heater for Tropical Fish Tanks

An aquarium heater is to keep the water temperature of a fish tank at a stable, desired level. It is an essential equipment if you have or plan to have tropical fish.

Do I need an aquarium heater for my fish tank?

As long as you have tropical fish, there is no doubt that you need a heater.  You fish can’t thrive without it.  They will be sick or even die. If you do not want to get a heater, please do not get any tropical fish because they will not live for long.  The aquarium heater is as crucial as the fish tank and aquarium filter.  

Why do I need an aquarium heater?

Depending on the actual species, most tropical fish must live in water at temperatures of 74~82F (about 23~28C) to survive. When the water temperature is below the ideal level, some tropical fish will become inactive and even refuse to eat. Before long, these fish might starve to death if not frozen to death first.  

In the case, your fish might still eat and act normally.  In the long run, lower than ideal water temperature can weaken their immune system.  Weaker immune system will cause the fish to become sick and die.

My room is warm enough now. Do I still need a heater for my aquarium?

Unless you live in a tropical area, you will always need a heater for your tropical fish aquarium. In most areas, the hot summer might provide high room temperature to keep the aquarium water warm enough for the tropical fish during the day, but at night the room temperature might be a lot cooler. Likewise, if you do not live in a tropical environment, it is unlikely that you have a high enough room temperature all year long. The limited water in an aquarium can cool down at a rapid rate when the air temperature is low. Tropical fish dislike massive swing of water temperature because it can shock and kill them.

Will an aquarium heater overheat the water in a fish tank?

No, unless it is malfunctioning or if you have the wrong settings, all aquarium heaters are designed to turn off by themselves when the desired temperature has reached They will automatically turn on again when the water temperature is getting colder than what you set.

Tips for Choosing and Using an Aquarium Heater for Your Fish Tank

1. Not all aquarium heaters are submersible
Please check the specification carefully on the heater. While some of them can be completely submerged, others have a marked water line for you not to have the water above it. We recommend everyone to use full submersible heater for your aquarium. Once again, check the specifications when you buy.

2. Not all aquarium heaters are adjustable
While some aquarium heaters give you the freedom of setting a temperature by yourself, some cheaper low model heaters have only one stock setting and it canโ€™t be changed at all. Those non-adjustable heaters will always have the same temperature setting on them, and it is recommended to buy adjustable heaters so you have the freedom and option to set your own water temperature.

3. Always use a thermometer with the aquarium heater
Do not trust the settings on the heater. Even for the most reputable brands, some exactly same model of heaters might result in different water temperatures when you have the same setting. It requires you to monitor the actual water temperature with a thermometer and adjust the settings on the heater accordingly. The thermometer should be placed on the opposite side of the aquarium heater in a fish tank to get a more accurate reading of the water temperature.

4. Do not let the water go below the minimal water line on the aquarium heater
Some non-submersible heaters have a line for you to keep the water below it.  All aquarium heaters also have a line for you to keep the water above it. If the water does not reach this minimal level, the heater will remain on because the air around it is cooler. Due to the fact that the air cannot transfer heat as quickly as water, the heater will quickly burn itself out if it is exposed in the air while it is on. This is more likely to happen during a water change for the aquarium, and it is recommended to turn the heater off before a water change to avoid damaging the heater as well as a possible electrical shock.

5. An aquarium heater should be installed near where there is a good water flow
Water can transfer heat fairly quick.  However, a large aquarium without a good water flow can still result in an uneven water temperature in different parts of the fish tank. It is unhealthy for the fish. You should always have good water current in your aquarium. The best place to install the heater is near either the intake or the outflow of your aquarium filter system. You may also place an air stone directly below the heater to have the air bubbles bring the water up around the heater.

6. Too low or too high wattage aquarium heaters are not suitable
All aquarium heaters have the manufacturer recommended tank size rated on them. When a heater does not have high enough wattage, it might never bring the water to your desired temperature.

Typical Aquarium Heater Specifications

Below are some of the usual aquarium heater specifications (They can vary for different brands.):
50 watt for fish tanks of 7~15 gallons
75 watt for fish tanks of 15~20 gallons
100 watt for fish tanks of 20~40 gallons
150 watt for fish tanks of 40~75 gallons
200 watt for fish tanks of 75~105 gallons
250 watt for fish tanks of 105~150 gallons
300 watt for fish tanks of 150~250 gallons

The above shows that manufacturers recommend 2~5 watts for each gallon of water. One thing I have noticed though is that good water current in the fish tank can compensate for the lack of wattage.  To a certain point anyway. Once after I had replaced my EHEIM Jager Aquarium Thermostat Heater 100W with a Rena SmartHeater Aquarium Heater 50W in my 40-gallon aquarium (because the Jager was too bright for my aquariumโ€™s black background), the water temperature was stable because of the excellent water flow. The only difference is that the 50W heater has to stay on for a more extended period than the 100w.

Of course, if the fish tank is way too big for the heater it will not work. For example, if you use a 50w heater for a 180-gallon or even 250-gallon tank, it might still not be enough even if you have an excellent water current in the fish tank.

All aquarium heaters will turn themselves off once the water has reached a predetermined temperature.  However, if you place a too high wattage heater in a too small fish tank, say a 300w heater in a 5-gallon tank, this might result in the heater being too hot to touch for the fish. Another downside of having an excessively high wattage heater for a too small tank is the heater will heat the water up to the temperature you set way too fast. It will result in too frequent on and off of the heater. The top reason for most break-downs of aquarium heaters is the result of too numerous on and offs. It can mean a shorter life for the heater if the heaterโ€™s wattage is too high for the fish tank.

Installing a proper aquarium heater in your tropical fish tank is crucial for the well-being of your tropical fish. You should check the water temperature every time you visit your aquarium.  Make it a part of the routine to notice any problem or malfunction in time. The electricity bill will only be a few bucks a month because the aquarium heater will not be always on. It is worth the cost.

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76 thoughts on “Aquarium Heater for Tropical Fish Tanks

  1. Hi I set up new 150 gallon tropical tank nd all fine ATM but my heater has water bubbles on them so is it ok to leave it as it is or should do any thing to get rid of it. No fishes in the tank ATM.

    • It is normal to have a lot of tiny bubbles appear all over the places in a newly set up aquarium. Leave them will be fine. They will disappear soon enough.

  2. Some heaters turn off by themselves after a while but how long do you need to wait before you turn it back on cause the water will go cold again won’t it?

    • All heaters will turn off by themselves when the temperature you set has been reached. The key is to have good water circulation in your fish tank. So the heater will make sure the entire tank has the same water temperature instead of just the water around it.

    • Hi Jay,
      I have a 50w heater working just fine in a 40 gallon tank. It translates to 1.25w per gallon of water.
      3x300w = 900w total, and for a 600 gallon tank it is 1.5w per gallon of water.
      I see no reason why not. It should work just fine (as long as) you have sufficient water flow in your tank. It is also the best to spread these three heaters out and place them under direct water flow near the filter or pump. The bubbles form the air pump will also raise the water so it is also a good source of water flow.

      Good luck! Please do share your aquarium with us on the forum. ๐Ÿ™‚

  3. The temp in my tank is 26 degrees and still all my fish are hanging a round the heater and some are dieing what might this be ???

    • Do you have sufficient water flow around the heater and in the fish tank in general? If you don’t have good water flow in the tank, the temperature will be different at different parts of the tank. You can increase water flow by placing the heater near the filter outflow, or place an air stone underneath the heater.

  4. I have a closed in porch that I am thinking of setting up an aquarium for tropical fish. However, the room does not have heat or AC. If I have a heater on the aquarium for winter is that suffiicient? The room is insulated and temp gets down to maybe 30 and sometimes but rarely below that. In the summer we will have a fan and open windows. Doesnt get too terrible hot though. Your thoughts pleass?

  5. I have just set up a 8 gallon aquarium with a 15 watt heater. Is that too little of wattage because on the package it says for 5-10 gallon tanks it was a “great choice 15 watt aquarium heater” please help

  6. my water does not seem to get warm . ive got a Jewel , 180 litres of water , how long should it take to get warm 1 2 4 or how many hours .the water is very cold from start , just bought it . please reply asap thanks Paul .

    • Hi Paul,
      What is the wattage on your heater?
      Make sure you have good water circulation in the tank in order to heat up the water evenly in your aquarium. Sometimes the water circulation is more important than the wattage. The 50w heater I used in my 40 gallon tank is “underpowered” by the heaters’ rating, but it does its job fine at keeping my water at stable temperature because I have good water circulation in the tank from my canister filter. If you have more questions, feel free to use our forum.

  7. Hey, I know that this will Sound crazy, but I have two tropical fish (used to have four) in with some non-tropical fish, they are in a ten gallon tank, they already have a filter, this is my fist REAL fish tank. I also have two beta’s in their own tanks, but my problem is that I don’t know if I should get the heater or not because 90% of my fish are not tropical, the guys at PetSmart must think I’m crazy by now… But should I get the heater?

    • Hello Raven,
      It is highly recommended to separate your tropical fish from non tropical fish before you get a heater.
      Obviously, a heater will be too much for non-tropical fish, but the tropical fish needs it.

  8. I just purchased a7.5 watt aquarium heater for my 2.5 gal tank with molly fish. This heater does not have a temperature set control. Will it overheat the water if I leave it on overnight? Nowhere does it tell me whether it is to stay on at all times or not. In scared to overheat the fish! HELP

    • Hi mmg,
      Some heaters are indeed not adjustable, but they do have a built-in default temperature setting. All heaters (adjustable or not) will automatically turn itself off once the temperature setting has been reached. No worry there. ๐Ÿ™‚ However, you definitely need a separated aquarium thermometer to monitor your actual water temperature.

  9. Hello. So I bought 4 fish recently (2 angels and 2 Rainbow Sharks) both tropical fish. I put a small heater in the tank, and at first they seemed fine. Today, my room got into the low 80s, and the fish kept swimming up to the top and spending their time there. Three died shortly thereafter. I quickly unplugged the heater, and the last one seems to be back to normal (it is one of the sharks). Should I plug the heater in again? Is there something I can do to prevent this?

    • The water temperature itself shouldn’t have killed the fish. It is the fact warmer water holds little oxygen. Do you have an air pump? If not, get yourself an air pump and problem should be solved.

  10. Hi I’m setting up a huge tanks 9 in total
    In the 9th one I will have a heater
    Yes there will be 2 pumps pumping water to and from the other 8 but I’m worried about the temperature
    Will it heat all the tanks up with just that one Big heater maybe the 300watt please help

  11. I agree with the “don’t rely on the heater thrmometer”. Some of my fish were dying and wasn’t sure why. All the chems were correct etc. but the water always seemed warm to the touch. So I tried a personal themometer and found my 79 F water was 92F! I disconnected it right away and went out and bought a new themometer. Now I use the same heater but at a much lower level and watch both Temps.

  12. I purchased a 64 litre tank a couple of days ago and there are obviously no fish in there at the moment.

    This is my first tank and I am so worried that I will not get the temperature right before I put the fish in. Should the water feel warm to touch or will I still find it slightly cold?

    By reading other comments I have seen that I should put my heater next to the filter to ensure this is heating the whole tank.

    Just now they are at opposite ends of the tank to each other, however it is not an overly big tank, should I move my heater?


    • Get a separated thermometer to check on the water temperature. Yes, it is a good idea to put the heater next to the filter intake or outflow. As long as you let the fish to get used to the temperature slowly when you add them to the tank, they should be fine even if the temperature is a little warm or chill.

  13. is it ok if my heater is still in the tank even this is unplugged?i plug it if theres a bad wether,,,thnks ๐Ÿ˜‰
    more power!

  14. Hi we got a 110 litre tank and got a heater for it. When we switched it on the water was freezing cold we set it to heat to 24 we left it over night but I dont trust the thermometer the water feels slightly cold to touch is this right? Is that how it should feel by touch. The reason why dont trust the thermometer is this is our 3rd now but we got one that stuck on glass like a strip but it was saying it was over 25 but the heater broke so had to get another one. I wanted to know how long to leave the heater before knowing if its not working? Obviously we new at this I just dont want to kill fish off when just starting out with tropical tank

    • A lot of heaters have some kind of lights to indicate if it is turned on or not. You need to make sure there is decent water flow in the fish tank especially around the heater. While thermometer can’t be trusted, you can get several of them to place at different spots in th fish tank to get an average reading which should be closer to the real one.

      Human body temperature is 37C, so of course the water will always be cold to touch for us.
      P.S. I’d aim for 26C which is around 80F.

  15. Hi, I have just set up a small tropical 22 litre tank for eventually 6 neon tetras. The tank is cycling, with aqua start and tap safe and heater appears to be at 26 degrees, and the filter appears to be circulating the water. I also bought a thermometer which I have placed as instructed on the outside of the tank under the water line. The temperature on this agrees with the heater temperature. My question is, should the water feel warm to the touch or should it remain coldish? For 26 degrees it does not feel warm. I am puzzled as I thought the water would feel warm, obviously not too hot to overheat the fish but warmer than it is. The heater and the pump are in a separate part of the tank at the back (aqua nano 30 (22litres). Is this correct or do I need to worry that something is not working correctly? Not intending to put fish in it for at least 2 weeks. It has been cycling for 3 days now. Please can somebody help me. Thank you.

    • My main tank is 80F which is about 26C. Human body temperature is 37C, so even at 26C the water should be cold to touch. It is normal.
      Fishless cycling usually can’t be completed for 4~6 weeks unless you use live bacteria products such as Tetra SafeStart along with a source of ammonia.

  16. I have a 20 gallon tropical fish tank (and one gold fish) and my heater died months ago. Is it safe to put a heater in the tank while the fish are in it? I heard having the water slowly heat up could kill myou fish. What should I do?

  17. I recently purchased a beta falls aquarium. I believe the tank is around 4 gallons. However, I had a hell of a time finding a 25 watt heater with a programmable temperature gauge, that was small enough to fit in the reservoir, and available at my local pet stores. I purchased a 50 watt heater (glass encasing) that is adjustable and it fits great (it’s impossible for the fish to come in contact with it, so not an issue). I set the heater at 78 degrees for my betas, I want to be sure it’s not going to overheat the water. Even though it has more watts than needed, it will still shut off at the desired temp correct?

    PS…I just need this to last the winter, so I’m not concerned if I have to replace it next year after the TX summer.


    • Aquarium heaters are designed to automatically turn off when the water temperature has reached the desired setting. So even if the heater is rated higher than your tank size, it will be fine as long as you have decent water current in the tank.

  18. Guys I have a 7.5 gallons or 11 gallons tank . Which watt heater should I purchase I havave 2 Mekong catfish and 1 goldfish . But they r all fine from 2 yrs until yesterday the shark [Mekong catfish] got covered with white structure on its whole body iam scarescared because it’s not that aactive from 2 DAYs and now I am sure it’s not white spot disease so tell me what to do with shark and heater plzzzzz reply fast

  19. I have a 35gallon aquarium with an 150w heater in it is that right size heater
    got airpump running underneath it and its near to external filter output flow.

    • You certainly can use a 150w heater in a 35 gallon fish tank without a problem. Your setup has made sure of it since you have good water flow around the heater and in the tank. Good job! Just make sure you use a stand alone thermometer for temperature monitoring.

  20. Hi,

    I have recently purchased a 140litre aquarium and am in the process of getting all the bits together. My heater is a AquaTropic Aqua Therm 200W. I have noticed some white powder looking substance, internally at the end of the glass tube. Do you have any ideas what this could be? And is it normal?

    I have no water in the tank yet, so have not tested it.


    • Hi Chris,
      I am unfamiliar with your particular heater, and I have not seen it with my own heaters. If the heater is still working properly, I wouldn’t worry about it. Make sure you use a stand alone thermometer to check the water temperature.

  21. Hi I have a 300watt heater but it doesn’t seem to be warming my water anymore but the light is on its a 240 liter tank

    • Do you have a separated thermometer to confirm it? You also need good water flow in the fish tank to evenly distribute the heat.

  22. Hey, im going on holiday soon for 3 weeks (or more) i was wondering how long would the water quality last for my 105 litre tank ( 9 mollies and 6 black ruby barbs ) ?

    • I suggest you do a large partial water change of at least 30~50% just a day before you leave. Fish can go on for a few weeks without food no problem. Without feeding, the water quality won’t go down as quickly. If you have someone to come in and feed the fish, let them do it no more than once every 2~3 days. Or even just once a week will be fine. Feed them minimal food that they can finish all within a minute. The less the better. If you have any other questions, please ask them on our forum. ๐Ÿ™‚

  23. I have a 75 gallon tank, and the temp is around 80 degrees F. I have some Dalmatian mollies among some of the fish in the tank, and one of them was hanging around the heater and then it got pop eye, we put it in a hospital tank and he got better and then he died a few days after being put back in the tank, now the other mollie is hanging around the heater. any idea why?

    • Do you have good water flow in the fish tank? The heat might not be distributed evenly throughout the tank if there is not good water flow around the heater. Please use a stand alone thermometer placed far away from the heater to monitor the water temperature. As for the popeye problem, it has nothing to do with the water temperature. Popeye in fish usually occurs when the water quality is not so good. It is difficult to treat once it has already happened. I would do partial water change of 30~50% along with extensive gravel vacuuming to improve the water quality. Cut back on feeding is also useful in keep the water cleaner. Excellent water quality along with not to overfeed usually solve most of the problems. If you have more questions, please use our forum. ๐Ÿ™‚

  24. I just bought a small beta tank and a 7.5w heater that does not have a temp control on it. I was wondering if this is going to be to warm for my little tank?

    • Without knowing the actual fish tank size, it is hard to say. In most cases, as long as you have good water flow in your fish tank, it is fine. Aquarium heaters are designed to automatically turn off when the set temperature has been reached.

  25. Hi I have a 30 litre biorb halo tank. But recently my fishes are dying. I don’t know why as the water I checked is fine and The tank has a built in air pump. Though my thermometer shows temperature at 80/82 f and my local pet shop guy said it’s too high. But the thing is my heater is set at the lowest settings at 22. So I thought my thermometer is broken and I got a new one. Unfortunately it wasn’t the thermometer as the new thermometer shows the same temp. What should I do? Is my heater broken? Please help

    • 80~82F is just fine for most tropical fish species.
      When you said the water is “fine”, do you mean the ammonia and nitrite are both 0ppm? What is the PH? Nitrate concentration?
      What filter system do you have?

      30 liter is also not very big. I wonder how many fish and what species do you have in there?

  26. Hi, i have 3gal tank and a slim filter , without air pump. I recently installes a 10w submersible heater, i was wondering if i nees to install an air pump? The heater is preset type ans not adjustable, so am a bit worried it would kill my fish. The tank houses 2 small parrots at the moment. Looking forwarf to yoir advice. Thank you.

    • It would be in the best interest of your and your fish to get a much bigger aquarium. The 3 gallon is way too small to have stable water conditions, as well as too small in size for your fish species.

  27. do i need to have my filter running for 3 days before putting in fish, or can i buy fish and heater at rhe same time? (the tank is set up and the filter has run for 3 days)

  28. Hi, I have a 10 gallon tank for tropical fishes 2 danios 1 barb I just bouhgt the heater is safe to put it in the tank with the fish in it? Because I don’t have any other thank to keep my fish! I’m a newbie!

    • Hello Ivan,
      Aquarium heater is meant to be staying in the fish tank while the fish are in there.
      Make sure you also have a running filter. ๐Ÿ™‚
      If you have any more questions, please come to the forum and post a thread under the fish section. ๐Ÿ™‚

    • Hi Roxane,
      It is fine as long as there is good water circulation in the fish tank. However, a lower wattage heater will do just fine.

  29. So, I’m going to put my oscar fish in this new 40 gal tank because he is out growing the 20 gal tank. So I am thinking, what type of heater should I get for my oscar fish so it the temperature reaches 26c/79f?

    • Hello Brooke,
      The same heater from 20g tank will work just fine as long as there is good water flow in the fish tank. I have used heater as low as 50w for a 40 gallon and it was just fine. If you have any further questions, please use our forum.

    • There is no need for two heaters. What you need is good water flow in the fish tank. You can create water flow through the use of filter system and air stones. For example, by placing an air stone right under the heater, the water will rise around the heater continuously as long as the air pump is on.

  30. I have a 5 gallon tank with a betta and a couple chinese alge eaters. With summer coming and the air conditioner running, I bought a small heater that is for 2 to 15 gallon tanks. The water temperature is currently at 70 degrees, can I put the heater in the tank with the fish currently living in 70 degrees water? Will warming up the water hurt them or make them sick?

    • Hi Sue,
      Aquarium heater will automatically turn itself on and off according to the water temperature in the tank and the setting on the heater.
      By the way, your fish tank is not big enough for Chinese algae eaters. The 5-gallon tank is perfect for just a betta fish.

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