In-line Aquarium Heaters - Planted Tank Mates

In-line Aquarium Heaters


In-line aquarium heaters (external) are a popular choice for those who want to visibly reduce equipment inside their tank.  They are placed on the outflow of your filter and hidden from your aquarium.  Although they take more effort to set up, they’re praised for their ability to maintain warm, tropical water temperatures.  In comparison, traditional aquarium heaters (internal) are placed inside the tank with an exposed electrical cord.  You’ll often see them placed horizontally or at a diagonal in order to heat the water more evenly.  

In-line / External Aquarium Heater – available on Amazon


Internal heater placed diagonally

General Benefits 

One of the greatest benefits of an in-line aquarium heater is the even heat distribution.  Water is passed through the heater chamber and warmed up before it enters your aquarium.  Instead of hot, warm and cool spots in your tank, inline heaters create a consistent overall water temperature. 

In-line (External) Heater Diagram


Internal Heater Diagram

Having an inline aquarium heater also prioritizes the space and aesthetics of your tank.  It allows you to focus on the aquascape and not the electrical device in the background.  Especially if you have a small tank, you’ll want to minimize the clutter inside so there’s more room for plants, hardscape and livestock.

Aquascape with minimal devices inside

The extra benefit to getting the heater out of the tank is livestock safety.  Although rare, fishes can get burned when caught between the internal heater and glass.

Internal heater and fish

The Downside

In-line heaters only work with canister filters or sumps.  It needs an outflow tube to connect to.  They do NOT work with hang on back filters or sponge filters.  Depending on your setup, this can be a limiting factor for heating up your aquarium.  You can read more about the different types of aquarium filters here

These devices can be large and take up space in the cabinet.  You’ll need to provide extra room for more tubing to transition between the filter, heater and the outflow device.  This device can also reduce the outflow pressure of your water.  It’s recommended to minimize the tubing length as well as possibly upsize the capacity of your filter to ensure sufficient water flow is provided in the aquarium.  You can always reduce the pressure output, but it’ll be hard to turn it up if it has to pass through other devices before your tank.

Last but not least,  another device with tubes means more potential for leaks.  If this is placed under your aquarium cabinet, you can always get an aluminum tray or something to place beneath to catch any water.  Use a leak detection device that sounds an alarm when it detects water.  There are several options that even connect to your phone and send you an alert if water is detected.

smart leak detector – available on Amazon


There’s a great DIY tutorial on youtube from The King of DIY.  He makes his own external heater for a fraction of the cost using an internal (inside fish tank) heater and PVC fittings.  This has been tested and worked for years, but the uniseal can wear out over time and leak.  Be sure to add aquarium sealant around all edges and occasionally feel the fittings to make sure it’s dry.  This DIY setup takes a little time, but gets the job done well for anyone on a budget. 

the king of DIY external heater – youtube

All things considered

In-line / external aquarium heaters provide a consistent temperature for your aquarium.  It takes one extra device out of your tank leaving more room for livestock, plants and prioritizing the aquascape.  It does, however, add an extra device and tubing under your cabinet.  Water flow may be reduced, so consider up-sizing your canister filter to ensure sufficient outflow pressure is provided.  There’s a higher upfront cost to an inline heater, but there are reliable options for making your own.  

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