Aquarium Heater Suggestions for Fluval Spec Aquariums

What I like best about the Fluval Spec line of aquariums is their ‘all in one’ appearance, where the display tank, lights, and filtration system are all in one box. When setting up our Spec V aquarium, it was only natural to try to find a heater that would fit into the pump section and be totally enclosed. In my research, I found a few heaters that would work well. I will talk about these and also go into some other aspects of aquarium heaters.

Option 1: Fitting a Heater in the Fluval Spec Pump Section

I think this is the preferred method for putting a heater in your Fluval Spec – it keeps things tidy and clean in appearance.  Things get somewhat tight in the pump section of Fluval Spec aquariums, what with the pump and outlet tube traveling up from the pump occupying part of the space. I found that there is enough room for a heater, but you have to look for ones that are small enough.

Hydor Theo 50 Watt in a Fluval Spec V Aquarium pump section

First of all, you should be looking for a submersible heater; one that can be fully dunked. Aquarium heaters are predominantly of this type. Second, if it comes with suction cups for mounting, strongly consider not using these. It is generally too tight to get the heater mounted with them on. When you shove the heater down into the pump section, it will get wedged somewhere between the glass and the outlet tube.

For specific brands, I have found a handful of aquarium heaters that I know fit the Fluval Spec V. Note that the pump section in the Spec V is a bit smaller than the Spec III, so it is somewhat harder to get a heater to fit in the larger capacity aquarium. Three that I know fit are:

There is another that will fit in the taller Spec III (but will not fit in the smaller pump section of the Spec V):

It’s important to remember that if you decide to put the heater in the pump section, you need to modify the flow tube with holes to allow water to flow around the heater. In a stock setup, water is largely stagnant in the pump section as it enters the pump at the bottom and travels up the flow tube and to the return nozzle. With no water flow around the heater, it will turn on, quickly heat up the stagnant water in the pump section, then quickly turn off. If you modify the flow tube with holes, some of the water will flow arond the heater to give it a chance to accuratly detect the water temperature and react accordingly. I have a separate page on how to perform this modification.

The Hydor Theo 50 watt is what I use in our Spec V. It has been very good, used in conjunction with my Temperature Controller.

Hydor Theo 50 watt prior to installation in aquarium. shown with suction cups for mounting

Option 2: Fitting the Heater in the Main Display

If you don’t want to deal with some of the challenges of fitting the heater in the small pump section or think that accuracy will be compromised with the heat source outside space where critters live, you can always put the heater in the main display. The only challenge is how to get the cord from a submersible heater out in a clean manner. I would suggest mounting the heater on the glass adjacent to the return nozzle. The cord might be routed up and over the baffle. From there you can route the cord over the filter or pump section and out the hole where the pump cord exits the acrylic top. The disadvantage to this is that the acrylic top will probably not close fully over the cord and may stick up a bit.  An alternative would be to route the heater cord out the main opening below the light.

If you decide on putting the heater in the main display, consider the Cobalt Aquatics Neo-Therm. It has accurate and reliable electronic controlls. It is slightly too big to fit in the pump section, but would be small enough to not be very noticible hiding in the display tank.

What Wattage of Aquarium Heater do I Need for my Spec?

The technical answer to this is that depends on many factors: how much internal heat gain from your equipment/lights, how cold you keep your house, what temperature you keep the aquarium at. However, if the aquarium is controlled by a thermostat, you can oversize a bit and cover a wide range of conditions.

As a general rule, the 5 gallon Spec V aquarium can have either a 25 watt or a 50 watt. I use a 50 watt and it certainly does not need more heating power. I think a 25 watt would have sufficed as well for our home.

For the smaller 3 gallon Spec III, I think a 50 watt is a bit much. A 25 watt heater is going to be your best bet.

One Type of Heater to Stay Away From:

In a search to find a heater that is small enough to fit in the pump section of a Fluval Spec aquarium, you will come across some models that are do not have an internal thermostat and are thus always ‘on’. They are small, simple, and usually have very low power of around 10-20 watts. I would avoid these. I’m not averse to using a heater that is a bit underpowered as I think that most heaters are oversized for the heat loss experienced in the winter. However, without the ability to adjust with changing conditions, you will not be providing your aquarium inhabitants with temperature stability. As the environment changes with light schedules and room temperatures, the heater will always be on and will not alleviate these fluctuations. I don’t see much benefit in them and suggest you find a way to control your heater, either with a model that has an internal thermostat or by an external temperature controller.

Accuracy and Reliability:

If you search through reviews of many aquarium heaters, you will find stories of satisfaction, but also some horror stories of heaters failing. It is failing while ‘on’ that is the real problem; cooked critters.

There are two ways to combat this problem. The first is to invest in an aquarium heater that has advanced electronics for the temperature control. The problem is that most heaters in this category do not fit in the pump section of the Spec. The Eheim Jager is a great example of one that does offer better reliability and control, but unfortunately does not quite fit in the Spec V.

The second way to avoid a run-away heater is to use a separate temperature controller to turn the heater on/off. I used to use the DIY aquarium temperature controller I built, but I now use a pre-wired temperature controller; they both work great.  It has a separate temperature probe located in the filter section to accurately detect the tank temperature. It then fully controls the on / off function of the heater, thus eliminating much of the risk for overheating. I built mine for around $30 – see how I did it here. If you don’t want to go the DIY route, there are some aftermarket aquarium controllers that can do the same thing, but they cost more.

A disadvantage to both the DIY temperature controller and a reefkeeper type controller is that they take up space outside your aquarium. I have dealt with this by putting everything into a storage box to hide it.


Final Tips for Aquarium Heaters:

Make sure to verify your aquarium heater’s thermostat with a separate thermometer. Preferably, an accurate one. It is not uncommon for the more inexpensive aquarium heaters to be off by a few degrees, where a heater set on 76F will actually hold low at 74F or may be high at 78F. The actual number is not that important. What is important is that you can realize where to set the dial to achieve your desired temperature and that the heater be consistent.

Finally, if you are not going to spend the extra money on an external controller or a model with electronic controls, consider selecting a heater that is slightly undersized. This will at least lessen the carnage if it fails ‘on’ and will give you more time to notice the problem and react before things get out of hand.

Having said that, I still wholly recommend utilizing a good temperature controller. Most aquarium controllers can also be paired with fan to provide cooling control. It adds peace of mind and greater consistancy for temperature making your fish and inverts healthy.

12 thoughts on “Aquarium Heater Suggestions for Fluval Spec Aquariums

  1. Just a quick question. Right now I have the Hydor 25w in my Spec V. I have the digital thermometer down in the filter block (as you suggested) and my readings are struggling to get out of the 74-75 range. My Spec III has the same set up(same heater) and does fine with readings in the high 70’s. I have done all the modifications (holes in the tube as I did in my III), but my question is do you think I should go for the 50w. The tank is in my finished basement, but as in most houses the basement is much cooler than the rest of the house. I have been trying for about 3 days to get this modification to work, but I’m worried that the heater staying on constantly means that it just isn’t powerful enough not to mention that can’t be good that it’s always on.


  2. I installed a 25 watt Hydro in a Fluval Spec-V. Inserted a thermometer in the pump-area as well, and another at the other end of the tank.

    I have it set for over 80, and it just keeps the tank at about 77, since set up – which was 4 days ago.

  3. So, there are no complications with the heater resting against the output tube (carrying the water back into the display tank) inside the filter / pump compartment? Sorry if this is a silly question, first time fishkeeper – just got a Spec V and I am setting it up.

    • It’s not a silly question at all. I worried about that myself. Rest easy – the outlet tube is made of silicone and can handle temps as high as a heater can produce. It shouldn’t hurt the heater either. Still, try to arrange things in there so that there is as much space around the heater as possible.

      • Thanks Nate! I am going to try to set everything up this weekend.

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