Heater Placement


The past week I have been thinking about heater placement in an aquarium. There is a lot of conflicting advice on the internet and in books. I am no kind of authority on thermodynamics but I thought I would post my thoughts on the subject as well as what others say. If you think you have a good idea on where the heater should be placed, please share in the comments.


There are three ways which heat can be distributed throughout your tank. These are conduction, convection, and circulation.

Conduction is the typical spread of heat from hot to cold for thermal equilibrium. When the water is warmed up, the water molecules get more kinetic energy and start bouncing around more and this heat energy spreads throughout the water. Water is a decent conductor of heat, but you are going to get most of your transfer from convection and circulation.

Convection is where the warmed up water becomes less dense and rises to the top creating convection currents. When your heater warms up water it rises to the top to be replaced with cooler water.

The main vehicle for heat transfer in your aquarium will be from the circulation of your filter. Ideally, your filter will mix the water thoroughly to prevent any hot or cold zones.


One commonality from all the suggestions I have read is to place the heater where there is a lot of flow to spread the heat evenly. This makes since so the warm water from the heater will be sucked through the filters intake a long with a cooler stream from the bottom of the aquarium.

In the book Aquariums for Dummies, it says to place your heater diagonally from top to bottom. I don’t get that. It doesn’t explain why either.

In the book The Simple Guide to Freshwater Aquariums, it suggests putting the heater horizontally just above the gravel near the filter Discharge. That is if it is a fully submersible heater. This mixes the hot zone from the heater with the tanks circulation water. It is also suggested to use two smaller heaters for bigger aquariums. This gives you back up if one fails and will also provide better heat distribution.

From the how stuff works website, link provided in sources, it mentions putting an air stone under the heater to push the dense cool water near the bottom up towards the heater.


It is really better to have a fully submersible heater rather then a partially submersible so you have more freedom of placement. If you have a heater that can only be submerged partially, it would probably be a good idea to put an air stone under it to help circulate the cool water.

Some submersible heaters do not last as long if you install them horizontally, but it is probably a good idea to replace your heater once a year any way. That way you can try and prevent a failure before it happens.

As you can tell from the picture of my 20 gallon aquarium above, I have a submersible heater placed somewhat horizontal near the filter intake. Don’t laugh, it’s a work in progress. This placement makes sense to me since I will be taking advantage of convection and circulation for heat transfer.

If I had a bigger tank, I would have two smaller heaters. One horizontal bottom right and one horizontal bottom left center. I would also consider putting an air stone below each heater for better circulation.


I really like the air stone idea. If you were to fancy one of those bubble wands or curtains, you could put it in front of the heater in the horizontal position and it would help to hide the heater. Good ideas, bad ideas? Let me know what you think!


Hargrove, Maddy, and Mic Hargrove. Aquariums for Dummies. Foster City, CA: IDG Worldwide, 1999. Print.

Boruchowitz, David E. The Simple Guide to Freshwater Aquariums. Neptune City, N.J.: T.F.H., 2001. Print.


So... what do you think? Please leave me a comment.


  • Vale: Nice post! Are there any heaters out there that work alongside the filter, like the way a pool heater heats the output from the filter before returning it to the pool.
  • big_sw2000: Craking blog
    @ Vale. My father has a filter, which holds the heater as well.
  • Sewerplant: Thanks! I have heard of people keeping the heater with the filter. I like this idea. It would be good for the biofilter also.
  • debcc: Good info! I keep mine horizontal but it was because they’re fully submersible and horizontal made for easier water changes. I will certainly keep this in mind when replacing heaters!
  • Vale: Thanks for letting me know Steve, I figured they must have something like that for aquariums too!
  • CarLey: Thanks for reminding me of proximity to filter or water movement! At the moment I’m only using a used heater in my cycling 15g, but I had it at the other end from filter! I will move it over by the filter.
  • chris w.: Good info, thank - I’ve got a bubble wand and now will consider moving it towards the heater for screening & circulating

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