Selecting the Proper Sized Aquarium Heater for Your Fish Tank

 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.