Will an Aquarium Heater Melt a Plastic Tank?

A woman and what I can only assume was her son were ringing up supplies for her son’s first fish at our local pet store. It’s a small shop; I couldn’t help but overhear the clerk tell the woman the heater they were getting would melt the plastic tank they were getting. Something smelled fishy to me, so as soon as I got home, I began to research.

Can an aquarium heater melt plastic? As long as the aquarium heater isn’t resting directly against the tank, a properly functioning aquarium heater will not melt a plastic tank. Plastic plants and decorations will be fine as well, as long as they’re not making constant contact.

Despite this, the woman and her son’s experience is relatively common. I found stories of well-intentioned sales clerks refusing to sell plastic tanks and heaters together and overly-vigilant eBay vendors everywhere. Why is this myth so prevalent, and might there be some truth to the misconception?

Why Won’t an Aquarium Heater Melt a Plastic Tank?

Generally speaking, if your heater is installed correctly and is in working order, you shouldn’t have to worry about it melting your plastic tank. It may seem counterintuitive at first, but there are two reasons why a heater doesn’t pose a problem.

First, and perhaps the most often cited rebuttal, is the fact that most aquarium water heaters simply don’t get hot enough to melt the kind of plastic tanks are made of. This is mostly true.

There are a number of different plastics used in fish tanks. Each kind can withstand a different temperature; the absolute lowest end of the heat tolerance spectrum is 149°F. We’ll go over identifying the kind of plastic your particular aquarium is made of later on; however, most aquarium heaters simply do not get hot enough to pose a risk to even the lowest tolerance plastic tanks.

With that in mind, however, it’s still best to avoid heaters made specifically for larger-sized tanks, such as titanium heaters and high-wattage heaters. These get much hotter than other kinds of heaters and pose more of a risk to plastic tanks. Most plastic tanks, however, do not come in a size that warrants either of these heaters.

The second major reason for why an aquarium heater won’t melt a plastic fish tank is water. Water is a fantastic insulator and heat distributor. Even slightly moving water will ensure your entire tank is heated evenly. The part of the tank next to your heater will never get hot enough to damage the plastic, even if there are only a few centimeters of space between them.

Other plastic fixtures in your aquarium tank such as plants and decorations will be just as safe so long as they don’t keep sustained contact with the heater.

How to Pick the Best Kind of Plastic Tank

Knowing what kind of plastic your tank is made of will help you choose the best heater to suit your needs.

Some plastics are more heat tolerant than others. Take a moment to check the packaging of your tank, or look for the number in the recycle symbol on the bottom of the tank. If the number is 2, your tank is made of high-density polyethylene (HDPE) and can comfortably withstand temperatures up to 149°F.  A number 4 means low-density polyethylene (LDPE), which tolerates the same temperature.

Number 5 plastics are made of polypropylene. These plastics can withstand a much higher 176°F. If you’re using a higher-wattage heater and still need a plastic tank, try looking for a polypropylene tank. A more thorough description of the kinds of plastics found in aquarium tanks can be found at SpecTanks.

Choose the Best Wattage for your Plastic Tank

The size of your tank dictates the wattage your heater needs to be to effectively heat the tank. TheSpruce has a great guide for choosing wattage based on tank size here (https://www.thesprucepets.com/aquarium-heater-size-guide-1381033). Be sure to choose the wattage of your heater as accurately as possible. Too low of wattage can overwork your heater, while too high of a wattage runs the risk of overheating the water, the heater, the fish, and your tank.

If you’re heating a larger tank or a tank that’s on the cusp between two different wattage recommendations, it’s always better to get multiple lower-wattage heaters than one higher-wattage heater. Getting lower-wattage heaters will ensure you don’t overheat your plastic fish tank; it will also provide backup in case one of your heaters goes out unexpectedly, and let you fine-tune your heating when the weather’s hotter or colder.

What Heaters to Avoid with a Plastic Tank

I don’t recommend using a titanium heater in a plastic tank because of the high temperatures it can reach. It’s also best to avoid using substrate heaters, especially in a smaller plastic tank, as the substrate doesn’t conduct heat as well as water.

Many smaller plastic tanks (such as betta tanks) come with their own heater, but these are generally cheap, not strong enough to heat the entirety of the tank, and more likely to break. Investing a bit more money into a better quality heater will ensure your fish have a better life and will save you money in the long run, as you’ll have to replace it less often.

How to Heat a Plastic Fish Tank with the Best Heater

The two heaters recommended for a plastic tank are fully submersible and partially submersible. Of the two, fully submersibles are generally better; they’re slightly more expensive, but they’re much more efficient since every part of the heater is in the water. This also means you’ll get more water moving around the heater – which means more heat distribution and even less of a chance of damaging a plastic tank. There are many different kinds of fully submersible heaters; you can learn more about heaters at FishTankWorld.

Partially submersible or hanging heaters hang from the side of the tank with the electronic part of the heating system sitting above the water. These are generally cheaper, as they don’t have to be fully waterproof, though they can be a bit more difficult to install. They’re also less efficient and run a slightly higher risk of overheating your aquarium walls.

Safety Measures to Take When Using a Heater in a Plastic Aquarium

In the end, the safety practices for a plastic aquarium and any other aquarium are exactly the same.

Be sure to read and closely follow all instructions when installing a heater. Many will have you put the heater in the water first, unplugged, to allow the heating element to adjust to the water temperature before starting it up. Similarly, a heater should be allowed to cool down with the water before being removed.

An incorrectly installed or malfunctioning heater may melt your tank, just as it can shatter glass. If your heater doesn’t fit correctly to your aquarium, is not working properly, or was turned on while it was out of the water, replace it. Immersing a malfunctioning heater in water can overheat or electrocute your fish, burn you, or start a fire.

Frequently Asked Questions

What heater is recommended for a small, plastic fish tank?

My top recommendation for a smaller plastic tank (anywhere from 7-15 gallons) is the Aqueon Pro Adjustable 50W heater. It’s fully submersible, has adjustable heat settings, and auto shut-off when it overheats, all of which are great features at a lower price point. It may not seem like much, but it has an aluminum housing instead of a glass one, as well. Glass runs a slight risk of shattering if its temperature shifts too quickly (for example, if one end is touching gravel). Aluminum is a safer bet if you’re working in cramped spaces!

Will heating a plastic tank poison my fish?

Polyethylene and polypropylene, the two most common kinds of aquarium plastics, are food-safe plastic and remain safe up to at least 149°F, depending. While they are safe for your fish for a few years, I recommend making the switch to a glass tank when you can for longer-term pets.

I need a heater for an emergency tank/quarantine tank/slightly questionable plastic container/bucket. Is it safe?

So long as the container is clean, you can attach your heater safely, and it’s made of the right kind of plastic – look for a recycle symbol with the numbers 2, 4, or 5 inside – your container is perfectly safe to put a heater in.

Why was I told I couldn’t put a heater in my plastic fish tank?

There’s a number of reasons, honestly. It may be misinformation; it may be that the person in question picked the wrong kind of heater, didn’t pay attention when their heater gave signs of malfunctioning or didn’t install the heater correctly. For what it’s worth, most of the stories I’ve heard about heaters melting plastic aquariums tend to be user errors rather than a problem with the aquarium itself.

Do I need a heater for my plastic aquarium?

This depends on the kind of fish you’re keeping and the temperature of the room you’re keeping your fish in.


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