There are some fish that are perfectly happy at room temperature, such as cloud mountain minnows, but you open yourself up to many greater varieties and better overall environmental control if you use a heater to keep the temperature of the water relatively stable.

Fluval M50 submersible aquarium heater

Recently I picked up a Fluval M50 heater rated for an aquarium up to 50L.  Since the Fireplace Aquarium is around 40L that’s about right.  The main claim to fame for this heater is that it can be relatively unobtrusive.  Because it is completely submersible it can be tucked away behind aquarium hardscape out of view.  The Fluval also claims to be narrower than most other heaters its size, again to be less visually obvious, and has a mirrored finish to “maintain the aquarium’s natural look by reflecting back the colours of its environment”.

The Fluval is narrower than my previous heater but is also longer – I think this is a wash visually.  Since the Fireplace Aquarium is taller than it is wide, there is lots of vertical space so a longer heater is not a problem.  A longer heater could be more of an issue in a shallower tank that has a geometry more similar to the Shrimphaus.

So about that ‘submersible’ piece… on Fluval’s website and all over the packaging you find this claimed.  Q:  Are M Series heaters fully submersible?  A:  Yes, M Series heaters are fully submersible.  However, when you RTFM you find, “For CSA/North America version: Water must not rise above the MAX WATER LINE indicated”.  Apparently the regulators forced them to say this but it really is submersible.  At any rate, I fully submersed it and so far so good.

Appearance and temperature control – both good

What about the mirrored surface?  At first I wasn’t sure this was a thing, but looking at the Fluval tucked behind the hardscape, in amongst all the plants, it is tricky to see.  It’s not invisible and if you put it out in the open it will stand out, but set back with other decor to catch the eye, the Fluval does a pretty good job of blending in.  The blue indicator light is very subtle – if you’re not looking for it you won’t notice it but you can see it when you need to.

Temperature control is fine.  You never really know whether the dial setting on the heater is correct, or whether the thermometer is – most likely neither of them is telling the true temperature.  That’s fine.  The point is you can find a temperature that works for you and the heater will successfully hold temperature at that point.  I have the Fluval holding the temperature at what indicates on the thermometer as 25C and that has been rock solid.

Care and maintenance

The Fluval has no over-temperature control so when plugged in it cannot be allowed to leave the water, for example during a water change.  It is good form to disconnect in-tank electrics such as heaters when doing any in-the-tank work in any event, just don’t forget.

HiDom heater
inexpensive aquarium heater


I keep a HiDom 100W submersible heater around as a backup. I used this one successfully for a couple of years.  It’s very affordable and does the job well.  Some things to be aware of:

  • The heater has no over-temperature automatic shutoff.  If you keep it plugged in while not submerged it will heat itself to destruction.
  • It has a temperature setting dial on the top with marked temperatures, but these are more like signposts than calibrated values.  Use an actual thermometer to measure the temperature of the water and turn the heater dial up a little to make it warmer, or down a little to make it cooler.

That all being said, this heater has been really great.  I destroyed one when I didn’t unplug it during a water change, but then I bought another one because other than that I was really happy with it.

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