What size should an aquarium heater be? (Based on gallon capacity)

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Unlike other animals that can regulate their own body temperature, fish must be entirely dependent on their environment.

This guide will help you find the best size aquarium heater for your aquarium so your aquatic companions can feel warm and comfortable even when it’s freezing outside.

To that end, I’m going to show you a formula you can use to measure how much horsepower you’ll need.

A rule of thumb for being on the safe side with pet fish would be to have 5 watts of heating power per 1 gallon of water.

However, playing around with formulas can be confusing, so I’ve also included a table to make it easier for you.

What is the proper size for an aquarium heater based on gallon capacity?

Back when I was new to fish keeping, I used to follow what the big pet store chains recommended for aquarium equipment.

Obviously, it seemed easier not to bother searching for the gear myself.

Needless to say, this caused a disaster in the first aquarium that I was personally responsible for.

A temperature fluctuation wiped out all my pet fish overnight.

If I had had a more powerful and larger heater in the aquarium, this could have been avoided.

So I decided to take matters into my own hands and do some proper research on the subject.

Here is what I found:

It usually takes about 2.5 watts of power to bring the temperature of a gallon of water 1 degree in an hour.

However, during a temperature fluctuation, 1 hour may turn out to be a little too long for our pet fish to wait. Most fish will be sensitive to these sudden movements within the temperature range of their aquarium. So I needed to approach math conservatively and calculate no more than half an hour.

So here’s how to determine the size aquarium heater you need:

Raising the temperature of a gallon of water by 1 degree Fahrenheit in half an hour requires 5 watts of energy. Therefore, an aquarium heater should give off 5 watts of heat for every gallon of water. This means that a 10 gallon tank would need a heater rated at least 50 watts.

After following this rule, I have had no temperature issues in my aquariums during cold weather swings.

I put together something like a summary chart that shows how much power you need depending on the size of your aquarium. Looked:

  • For 5 gallon tanks: 25 watt
  • For 10 gallon tanks: 50 watts
  • For 20 gallon tanks: 100 watts
  • For 30 gallon tanks: 150 watts
  • For 40 gallon tanks: 200 watts
  • For 55 gallon tanks: 275 watts
  • For 75 gallon tanks: 375 watts
  • For 90 gallon tanks: 450 watts
  • For 125 gallon tanks: 625 watts

A quick note on larger tanks:

Obviously, finding a standalone aquarium heater with great wattage would be difficult.

What you would want to do in such scenarios is get multiple heaters so that their total wattage matches your target.

Having two 300 watt heaters in a 125 gallon aquarium is perfectly normal and in fact I recommend doing it that way.

By having multiple heaters, you “spread the risk”, because if one fails, the other will still withstand the temperature in the aquarium somewhat.

My advice is to always have a new extra heater on hand so you can swap out a failed one in an emergency.

In my experience, temperature swings are one of the most common reasons fish get sick or die in new home aquariums.

What will happen if the power of your heater is too small for your aquarium?

A heater whose power is too small for your aquarium will have difficulty heating the water to the ideal temperature.

It may be able to reach the desired temperature at the site where you placed it.

But chances are the water will get colder as you move away from the radiator.

Ultimately, the heater will likely need to run continuously to have a chance of warming the entire tank.

This is likely to have adverse consequences and your small tank heater may burn out sooner than if you were using a properly sized one.

Often an aquarium heater fails when its element burns out, when the thermostat gets stuck, or when the thermostat won’t turn on.

If the ambient room temperature is relatively warm, your fish will not notice that the tank heater is not working.

But if it’s cold enough, as it can be on winter nights, you can lose your fish in less than a day.

That’s why, when asked, I always recommend going for a heater designed for tanks larger than yours.

Most good aquarium heaters have a thermostat that tells the heater when it’s time to stop working.

The thermostat measures the temperature of the aquarium via a sensor and when the desired result is reached, the heating stops.

More powerful heating does not mean hotter water. This means heating the water faster. This is what you should be looking for if you want to protect your fish from temperature swings.

Of course, the ability of fish to survive colder temperatures depends on the type of species they are part of. But most pet fish in the hobby are tropical species.

Also, it won’t hurt to put a lid on the tank to prevent heat loss and minimize any cold breeze ingress.

What are the different types of aquarium heaters?

There are many types of aquarium heaters that you can buy.

Almost all of them can be grouped into four types: immersible, submersible, filter and substrate heaters.

All of these types have slightly different characteristics.

This means that a particular type may be more suitable for your aquarium and your fish than others.

Here is an overview of the different types of home aquarium heaters.

Submersible Aquarium Heaters

As you might have already guessed, an immersible aquarium heater is one that you can submerge in your aquarium.

It usually consists of a glass or ceramic insert covered with a heating element and a glass tube.

While some immersible heaters are fully submersible, others can only be suspended partially from the top or side of the aquarium.

The latter type is known as hanging radiators.

A hanging heater will often have a clearly marked line beyond which you should not submerge in water.

One good thing about hanging aquarium heaters is that they are affordable.

In fact, many aquariums (by default) have them.

On the downside, they are not ideal for brackish or marine water as salt can enter the heater tubes and cause corrosion and/or electrical shorts.

Submersible Aquarium Heaters

A submersible heater is one that goes completely underwater in your aquarium.

Most of them are designed to look long, round and thin – like a pipe or a tube.

You attach the heater to the walls of your aquarium using clips or suction cups.

In any case, you can place it vertically, horizontally or at an angle.

Since a submersible heater stays entirely in the water, it is generally more efficient than a submersible heater.

This makes it ideal if you are concerned about your energy bill, but want to distribute heat efficiently throughout your aquarium.

I use submersible heaters in 100% of my aquariums.

Aquarium heater filters

If you don’t want a stand-alone heater, you can opt for an aquarium filter that comes with a built-in heater.

As far as I know, only cartridge and power filters can support this technology.

Either way, the aquarium heats the water as it passes through the filter.

This ensures that your fish get clean, warm water.

However, I do not recommend this type of heating.

It is better to have a separate unit specifically dedicated to water heating.

If it’s just an “add-on feature”, the build quality may be lower because that’s not the focus of the main product.

Substrate Aquarium Heaters

A substrate heating element consists of a cable with a heating element.

That’s why sometimes you’ll hear some people call it a heating cable.

In any case, the radiator is usually concealable.

You can easily hide it under the gravel or sand of the aquarium.

When the cable heats up, it heats up the gravel and radiates that heat throughout the aquarium.

Things to keep in mind

Using aquarium heaters is quite simple and straightforward. However, there are some things to keep in mind if you want to keep your fish safe:

  • Consider the type of fish you have when choosing a heater size for your aquarium. Some tropical species like bettas, angelfish and most tetras prefer warm, broiled water. Others like swordtails, barbs, and especially danios do best in cooler aquarium water.
  • It is better to install two radiators rather than installing one. This way, if one fails, the other can step in and heat the water. Just make sure their total wattage matches what your aquarium needs to maintain an ideal temperature for your pet fish.
  • You can use immersible and submersible heaters together. Placing an immersion heater at the top and a submersible at the bottom can help spread hot water more quickly and evenly. Of course, you also have the option of placing the immersion heater in a vertical position and the immersion heater in a horizontal position.
  • It is a smart decision to have a separate thermometer in the aquarium. This way you can estimate the water temperature independently of the built-in thermometers in your radiators. Always calibrate your aquarium thermometer (click the link for a YouTube video explaining how to do this).
  • Remember to always unplug the heater from your aquarium when changing the water. This will prevent it from overheating and breaking down when it runs out of water.
  • Get an adjustable heater. Non-adjustable aquarium heaters limit your storage options because they only support a fixed water temperature.

my last words

Ultimately, choosing the right size heater for your aquarium shouldn’t be too much of a problem.

Simply refer to the chart above for the best heater output for your tank based on its capacity.

In my experience, a good heater doesn’t have to be an immersible, submersible, filter or substrate type.

He’s the one who actually does the work. And if you find that one unit isn’t enough for your tank, go ahead and buy two.

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