Selecting the Proper Sized Aquarium Heater for Your Fish Tank


Soma, originally uploaded by J Koyanagi.

Q: MS wrote,

Great blog, I wonder if you could give me a little advice please.

I have a 5.5 gallon filtered aquarium with 1 betta. I am heating it with a 25w Hagen Elite which says on the pack it’s right for the volume of water. Trouble is even when I turn it down to it’s lowest setting I can’t get the temperature as low as I’d like. I understand perfect temperature is around 80°F but on it’s lowest setting the heater is holding the water at 86°F. Do you think my heater is broken or just not suitable? What would you recommend?

A: I too have found that most aquarium heaters tend to overheat rather than underheat. In fact I am dealing with the exact same thing. My new 100w heater, suggested for 30 gallons is keeping my water at about 80F even though it is set to the lowest it will go, which is 68F. This isn’t unusual as I recall my previous heater held my tank steady at 78F when it was set to just 70 degrees.

Aquarium heater manufacturers are making their recommendations based on actual water volume rather than the tanks maximum water volume. Once you add substrate, decorations and equipment much of the water volume is displaced. After taking into account all the displaced items in my aquarium my 30 gallon tank is really only about 25 gallons at most. In my case I probably would have been better off going with a 75 watt heater instead.

In your case, once you identify the actual water volume you may find that a 7.5 to 15 watt aquarium heater will better suit your needs. There is a little bit of guess work involved, which is why it is important to buy a heater with an adjustable temperature dial. Some circumstances out of your control can also affect the temperature including the ambient air temperature and your specific heater’s efficiency. Not all models are created equal so one 25 watt heater may warm your water more efficiently than another.

It can be a little tricky to find these mini heaters, at least here in the U.S. PetCo used to carry them but I couldn’t find them the last time I looked (quite some time ago). You may have better luck where you are. Either way, you can always find them online. You would think they would be more prevalent with the popularity Betta splendens and other small tropical fish.

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Christie F is a Betta splendens hobbyist that enjoys spending time caring for her fish and helping new betta keepers learn the ropes. More posts by:

9 Comments for this entry

  1. Christine says:

    I have found a small heater (brand Hydor) online – I'm pretty sure I've seen it in my local PetCo, too. It says it comes in either a 2-5 gallon 7.5 watt version or a 5-10 gallon 15 watt version.

    I've seen mixed reviews about it; haven't tried it yet (likely going to wait until winter to do so).

  2. Anonymous says:

    You can find a lot of online stores that carry the Marineland 10w heater preset at 78 degrees F…better option than the Hydor, which doesn't have a thermostat.

  3. simply Fish Tanks UK says:

    When it comes to choosing the right size fish tank, you may find it a little bit difficult to do your own research. Despite being difficult, it is possible to do. For example, you can examine the type of fish that you would like to purchase, namely their overall size and their swimming habits.

  4. simply Fish Tanks UK says:

    The reason for this is that most of the equipment you will need for the 5-gallon aquarium is the Hume used for the 10-gallon aquarium. Some of the equipment may cost more for the 5-gallon aquarium if it is not a popular item that is mass produced. Gallon for gallon, a small tank costs more than a large one. Therefore buying a very small tank is not really economical at all.

  5. Lilie says:

    I’m barely new to this and so I want the best for my new underwater friend.

    So then its best to buy a tank of how many gallons for him? (I only have one crowntail -maybe thinking of buying some compatible companions, not sure though)
    And also have a heater, and filtration?
    What about light?
    So live plants are better, which should I buy to start off with?
    and food, I give him flakes because he wouldn’t eat the pellets. I give him tetra bettamin™ is that good? I also bought freeze-dried bloodworms, but already read that this isn’t good for him… When you say frozen you mean like the fridge freezer right? (I know that he’s a great eater!)

    I REALLY REALLY want the best for my fishie(which by the way is still nameless right now lol, can’t really find a great name to suit him-he deserves the best!… Still under observation….-)

    • Christie F. says:

      If you do want to add tank mates, then that will be the driving factor to what size tank you buy. Fish have different tank size requirements. Typically, the minimum size for a community tank would be 10 gal. In a 10 gal you could add a few ghost shrimp or an Apple Snail or two. If you want more fish, like cory cat fish or Otos you might want a larger tank (15 or 20 gal) as they are social fish and live in small communities.

  6. shannon says:

    OMG,I meant in good intentions of warming my 10g community tank,and came home only to discover I had boiled my 2 beatiful bettas,4 guppies & sucker fish to death I was sooo devestated!!! A week later I couldnt take it,and despite being tight on $ I started over….I now have a lil heater thats intended for under gravel in 10g tank,but in my opinion it doesnt warm enough to notice the difference. I suppose after the holidays I will go on yet another hunt for a better heater.Any sugestions for one for a 10g tank?

  7. home heater says:

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  8. Ron says:

    Just make sure you get an adjustable heater. Those little preset heaters are junk. I use an aquacon 25 watt in my 5 gallon and a top fin 25 in my 2.5. Knock wood, no troubles yet!

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