Submersible Heater Placement: a Helpful Illustrated Guide

Written by Randy Martin

Something debated is where to place your submersible aquarium heater. There is a lot of different opinions on this and honestly I feel there are several ways to correctly heat your tank properly. But here is what I found that works best for me when you understand a few easy concepts.

Where to place the submersible aquarium heater? Place the submersible heater where you will get the best water flow to circulate the warm water. Place it horizontally and low in the tank a few inches above the substrate. If your heater isn’t designed for horizontal placement, then place it vertically next to your filter for best circulation.

The important thing is to be aware of the current of the water flow, and to work with what you got. Also make sure to read the instructions on your particular heater as they aren’t all the same. I will touch on some of the more key factors.

Where to place the submersible heater

First off I don’t know your individual setup and needs, and there is more than one way to get it done.

One of the most important aspects is that you have the correct size heater for your aquarium. That way it is powerful enough to heat all of your tank, but not over powered so it doesn’t create problems. I go into a lot of detail here about the best way to heat your aquarium and the correct size heaters you need.

Also a major factor is how good your water flow is in your tank. If the water is moving well and circulating through your entire tank and your heater is the correct wattage for your aquarium, then you will likely achieve a pretty regular temperature through out the entire fish tank.

In all cases you want to place your thermometer at the opposite end of your aquarium from your heater, so you can get an accurate reading of what is actually going on in your tank.

Placing heater horizontally

In general if I have the freedom, I am usually going to place the heater horizontally.

Towards the back of the aquarium, horizontally, and just a few inches above your substrate, gravel etc.

Submersible heater placed horizontally

The reason for this is that heat rises. So if you place your heater low, then heat will rise and better disperse around your tank. Also by being horizontal, you are covering more area across your tank to better help heat it up.

It’s important to note though that some submersible heaters clearly say in their manual that they shouldn’t be placed horizontally. You should always follow the instructions from your equipment above all other advice you can get from people on the internet that don’t know your exact setup.

I always recommend heaters that can be placed horizontally, you can read my article on the best and safest submersible and external heaters that you can rely on in your tank.

Important: Read the manual for your submersible heater to determine where it should best be placed. Search if there is any restrictions against placing it horizontally.

Keeping the heater low can make it easier to hide it and blend it in with your tank design, which is another consideration when placing your gear and trying to keep your tank looking beautiful.

Another benefit of having your heater horizontally and low to the ground is that you probably won’t even have to turn your heater off and back on during partial water changes, as the heater will stay constantly submerged. This could help lower the risks that we will get into in a bit. But to be safe it’s often best to go ahead and shut it off during partial water changes.

You should keep the heater at least a few inches off the bottom from your substrate so you don’t risk scratching or damaging the glass.

The importance of water flow

Okay so if your heater is horizontal and sitting towards the bottom back of your aquarium, should you place it dead center?

Centering the heater is something aesthetically pleasing in my mind. But really you should determine where you are getting the best water flow.

If the flow in the entire tank is good then you can place it center, or just about anywhere.

But a lot of times the water flow from the filter output could be hitting the other side of the tank creating a nice little current hole in that spot, so sometimes placing the heater there could make a lot of sense.

Water flow hitting submersible heater

Placing heater vertically

So is it bad to place your heater vertically? No, as noted above some submersible filters can only be placed vertically.

Also in tall slender tanks this is likely the only option for those too.

If you are placing the heater vertically it is generally recommended to keep it next to your filter, usually in the back corner of the tank. That way the filter intake is sucking up the warm water and distributing it back out into the tank, or the filter output is pouring past your heater, pushing warm water around the tank—or a combination of both.

Placing submersible heater vertically

The point is to work with what you got. Study your water flow and monitor your thermometer in different areas of the tank to understand what is going on.

Using two heaters at the same time

Sometimes depending on your size you might need more than one heater. Usually around the 50 gallon sizes and above you could start considering it. Or if you just aren’t getting good heat distribution and want the other end of the tank to be consistent.

If you use two heaters, place both at the farthest ends from each other. Of course as your tank size keeps going up so could the number of needed heaters. If that’s the case you could look into additional heating methods besides submersible heaters.

The nice thing about having multiple heaters in your tank is they help each other out. Since one isn’t having to do all the work, then they won’t be activated all the time wearing themselves out.

Another benefit is they can act as a fail safe for each other. If one heater malfunctions and dies, then your tank will at least continue to be heated by the other.

The draw backs is that you are doubling the risk that one might malfunction on you and overheat. Not to mention the additional power cost— however minimal.

Installing an aquarium heater for the first time

When you are installing a heater into your tank for the first time, you have to acclimate it slowly. You need to submerge the heater under the water and give it time to adjust to the temperature of the water. Do not turn on your heater when you first put it into the water.

Most instructions that come with the heaters will recommend at least 30 minutes of adjustment period under the water before you turn it on.

I would recommend even more than this. I would say when you first put your heater into the water for the first time, you should let it acclimate to the water for about 1 hour before turning the heater on.

This precaution makes sure that:

  • The glass tube and internal mechanisms will adjust to the water temperature, reaching a safer starting point to begin heating.
  • Makes sure the heater doesn’t heat up too fast resulting in a possible malfunction later.

Another thing you should have is a separate thermometer for your fish tank. So you aren’t relying completely on the thermometer on your heater.

Important: Checking the temperature of your fish tank is a daily task. This is something you should closely monitor every day. You need to know quickly if something is going wrong with your heater.

Risks of using a submersible heater

There are some risks that comes with using a heater inside the aquarium. One of the main risks being that sometimes it can malfunction and won’t turn itself off.

This will overheat your tank and can wipe out all of your fish. Too many fish keepers have experienced this.

As a result some people try to even avoid using heaters. I gave suggestions on how to avoid this also in the Best way to heat an aquarium article.

But don’t let that scare you, it’s just something to be aware of. Many others have been running aquariums for years and never had something like this happen, me included.

You can get one of the heaters I recommend here Best heaters for fish aquariums, I picked those heaters because they have the best track record in my opinion.

The important thing to remember is never remove your heater out of the water while it is running. If you need to remove it, unplug the power and keep it off for 30 minutes before removing it.

The same with partial water changes, make sure you turn off the heater before you remove any water, and give it some time to cool down just in case.

Doing water changes and forgetting to turn off the heater, then letting the heater come out of the water in a heated state is likely the biggest culprit for malfunctions.


Having seen a lot of different setups and hearing a lot of different opinions on this subject, I know that lots of different ways work. And it comes down to your individual setup. So it’s your responsibility to find what will work for your exact needs.

The things to look out for:

  • Make sure your heater is the correct size/strength for your tank
  • Know where your water is flowing and try and imagine how the heat will distribute in your aquarium
  • Use a thermometer to check the different areas of your tank
  • Monitor the water temperature in your tank daily
  • Check your heaters manual to ensure you are using it correctly

Okay, with all of this in mind you should feel pretty confident that you are probably doing things right when placing your heater in your tank. Keep experimenting and studying what works best for you.

Related Questions

Aquarium heater vertical or horizontal? Some aquarium heaters are designed so they can be placed horizontally, others can only be used vertically, refer to the user guide. If horizontal is allowed, then the ideal placement is horizontally and low in your tank a few inches above the bottom.

How to place heater in the fish tank? Place the heater into the aquarium water and attach it to the aquarium glass with the suction cups. Refer to the manual if it can be placed horizontally or vertically and if it’s completely submersible. Allow the heater about 1 hour to adjust to the water temperature before turning it on.

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