When it comes to buying equipment for our goldfish, if you’re like me you only want to get what’s really necessary.

And it’s easy to write off heaters as just an optional accessory…

… Especially if you’ve been told goldfish are coldwater fish and like things on the chilly side.


The TRUTH is, in many cases – you’re going to want one.

Here’s why.

Goldfish can live in cold water.

But the fragile immune system of the fancy goldfish is placed under more stress.

Fancy goldfish especially are more delicate than the typical pond goldfish.

As beautiful as they may be, they have a lower tolerance to disease and are especially prone to bacterial infections – external and internal.


Studies suggest that temperatures in the mid-high 70’s to low 80’s degrees F still allow for enough dissolved oxygen to remain in the water…

… But significantly reduce bad bacteria that can survive in it.

The best part?

Those temperatures promote the most growth in goldfish.

Unless your house stays that warm all year…

… You’ll need a heater to achieve that.

It’s a win-win!


Heaters also help ensure temperature stability in tanks under 20 gallons, which can fluctuate drastically depending on the time of day.

Constant fluctuation between warm and cold is stressful to goldfish.

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The Best Heater for Goldfish Tanks

I’ve tried a LOT of different heater brands over the years.

Some work well, some don’t hold the temperature properly, and others are a severe disappointment when they stop functioning after a few months.

(Replacing heaters can get expensive.)

Part of my long saga dealing with problematic heaters has to do with the fact that some heaters just are not made very well at all.

It’s like they were designed to fail.

(Did I mention a failed heater can be dangerous – especially if it explodes in the water and nukes your entire tank!!) :O


This is more minor when it comes to my evaluation of heaters, but it bugs me to have a long, ugly, distracting black thing in the tank.

So I was thrilled when I discovered the Fluval M line of heaters.

You’ll have to agree with me that it is very well made and high quality, unlike lots of other made-in-China heaters (it’s manufactured in Europe using a 16 point quality standard).

Get this:

The reflective design is the least invasive into your tank (besides an inline-heater which would need to be connected to a canister).

It has a blue indicator light that comes on so you know when its heating.

The dial is easy to turn and it holds the set temperature well.

If you’re big into your aquarium’s aesthetics like I am, I recommend replacing the standard black suction cups with clear ones.  Then your heater will be nearly invisible!

Sizes it comes in:

Check Price

How Many Watts Do I Need?

A good rule of thumb is 3-5 watts per gallon of water.


If the temperature around your water is significantly colder, it may require more watts to keep it stable.

An undersized heater can result in stressful temperature fluctuations – especially during the spring and fall.

It is not bad to go with more watts and err on the higher end if you are concerned because it’s easier to turn the temp down.

What about pond-type goldfish?

Most slim-bodied fish can live just fine in temperatures in the mid to high 60 degrees F.

They can even survive winters at even lower temperatures.

(It’s during spring that people have issues as the fish comes out of hibernation with a very low immune system.)

Going through a period of colder temperatures annually can also significantly add to their lifespan.

Tips for Using Heaters for Goldfish in the Winter


If you keep your goldfish in an unheated area such as a cold basement, garage or outdoor pond or tub and you live in a place where winter can get pretty cold, a heater is a good idea to help take the edge off.

While most heaters don’t go below 65 degrees (and say you want to keep the winter temperature around 50)…

Good news:

For aquariums, you can set a heater on a timer to go on every 30 minutes to keep it in the lower range. (It’s also a good idea to insulate the tank.)


If you keep your goldfish in a pond outside, a good quality pond de-icer can play an important role in preventing the water from freezing over and restricting oxygen levels to dangerously low levels (as well as prevent the water from freezing solid!).

Preventing Bad Bacteria Problems

Over time in the average aquarium, bad bacteria can get more and more of a foothold as the natural processes throw things out of balance.


You can’t totally get rid of them without killing your fish in the process.

They will always be there, it’s a question of how many.

More of them = greater chance of sick fish.

It’s something pathologists term “dose response.”

But one thing we can do (that’s a very simple and easy thing to do) is keep the water warmer.

Warmer water helps to reduce many types of bad bacteria such as aeromonas, columnaris and pseudomonas.

May fancy goldfish keepers find their fish struggle with chronic redness in their fins, red areas on the body, as well as extremely sensitive swim bladders (to name a few common issues) despite having perfect water quality, and it ties back to bacteria in many cases.

These problems can accumulate over time.

Our best bet to prevent bacterial issues is to try to reduce the bad bacteria count in the water as much as possible, while simultaneously establishing a strong colony of good bacteria.

(These are NOT the same bacteria that live in the filter.)


In addition to using a proper heater to maintain a warm, stable temperature…

…I use Dr. Tim’s Eco-Balance aquarium supplement which is a probiotic weekly to help out-compete bad bacteria in the fancy goldfish aquarium’s environment in my high(er) tech tanks.

I feel it really helps my longer finned fish without the need for a UV sterilizer.

And of course, regular water changes is a good way to keep things in their proper balance (though you may need to do lots of large water changes to help with bacteria, which can be time-consuming and make the water bill higher).

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For many goldfish keepers, heaters are an essential part of proper goldfish care.

What has your experience been with goldfish heaters?

Do you have any tips or tricks you would like to share?

Please feel free to leave your comment below!