Fluval Spec V vs Flex 9 – Which Model Wouldn’t Kill Your Fish

When it comes to Fluval fish tanks, the popular models are the Fluval Spec V and Flex series. Both models are great aquariums but have subtle differences. If you want value for money, I recommend Fluval Flex over Spec V. For an additional $20, you get a bigger tank, which comes with all sorts of benefits that I will cover in this article.

I will also tell you why the Fluval Flex is the preferred model of mine.

In particular, this article will cover:

  • whether Fluval Spec V or Flex is better in terms of value for money, stability of water, lighting options, and overall fish tank design
  • 4 reason why a larger fish tank is easier to maintain especially for a beginner
  • 3 modifications you might need to apply to either Fluval Spec or Flex if you do not want any accidents happening in the tank

Fluval Spec V vs Flex 9: Which is best for what

Value for money – Fluval Flex

For overall value for money, I recommend the Fluval Flex (click to see pricing and product specs from Amazon). The price difference between the two models is less than $20. For that, I encourage you to take the Fluval Spec, which has an extra 4 gallon of space.

This has 3 benefits:

  • Easier to stablize the water
  • More swimming room for your fishes
  • More aquascaping opportunities


Stability of water – Fluval Flex

It is pretty well known it is easier to stabilize a larger tank. If you don’t, here are the practical reasons:

  • Less temperature fluctuation, especially if you keep your fish near the window
  • Larger tanks allow for better lights to be installed, which in turn helps more plants to grow, which in turn help to keep the ammonia levels in check
  • Larger tanks also allows for multiple filtration systems to help stabilize the tank. For a smaller tank, due to the space constraints, you need to resort to chemical supplies to treat the water itself, which is more complicated.
  • Better algae eaters which solve the algae problem for you

The Spec V is a 5 gallon tank while the Flex has either the 9 or the 15 gallon option. That makes it much easier to hit the right water perimeters due to the reasons mentioned above.

Nano tank might sound easier but they are actually more prone to instability in water changes.


Better looking design – Tied

It is difficult to say which model has the better design.

Fluval Flex has the bow front fish tank design. Some people love it while other hate it. I am not a particular fan. For me, the traditional rectangular shape of Fluval Spec V appeals more.

It also depends on how you want to aquascape the tank.

The Fluval Flex can achieve the deep look with its larger footprint. In contrast, the Fluval Spec is more about a narrow but wide look.


Quality of stock light – Fluval Flex

The Fluval Flex is a newer model so it comes with the latest Fluval led lights. Although it is not as powerful as a dedicated aquarium light, at least it is strong enough to grow low light plants.

In contrast, the Spec V’s stock light is pretty weak. Even for low light plants, you will find they grow slowly. If you are getting the Spec V, I suggest you upgrade the lighting. (I will give a recommendation later)


Cons of each model

No product is perfect and the same goes for both the Fluval Spec V and Flex. I will list their weakness and make suggestions on how to mod your fish tank to overcome them

For Fluval Flex

  1. Danger to fishes: The biggest downside of Flex is the ease which fishes can swim out of the tank, either by the slats in the filter box or via jumping into the back of the tank. This is a headache but fortunately can be remedied by the following mods:

Mod tip: place a mesh over the slats and the entrance to the back of the tank


  1. Strong filter: The Fluval flex has a strong filter. Usually, that is a good thing but for small or nano fishes, it can be a problem. Betta with long fins, for example, can get their fins caught up by the filter. (Note: same problem exist for Fluval Spec as well but to a lessor extend)

Mod tip: Put a prefilter sponge over the filter. You can check out Aquaneat sponge from Amazon 


For Fluval Spec

  1. Strong filter. Similar problem as Fluval Flex so apply the same mod tip from above.
  1. Fishes can jump out. The Fluval Spec V has an opening near the light area where smaller fishes can potentially jump out.

Mod tip: place a mesh over the opening

  1. Lights are not movable. One of the slight troublesome thing about Fluval Spec V is that the lights are fixed. During changing of water or tank maintenance, the lights need to be removed. It will be easier if they can swing to one side.

Mod tip: none that I can think of

Do I need other accessories for Fluval Spec 5 or Flex?

Both models are considered all in one fish tank.  They comes with the all basic stuff like lighting, filter and wiring. Other things that you can consider include

  • Plants or decorations: Aquarium gravel or furniture are optional but they do make your tank looks that much nicer. Similar reasons for buying plants.
  • Measuring tools. If you do not have these, I recommend getting tds meter to check the water “hardness” and the API Freshwater Master Test Kit to test ammonia and nitrite levels in the tank.
  • Water Conditioner. For experienced fish owners, you know what to do when it comes to cycling your tank so this not necessary. If you are a beginner, you might want to consider water conditioners to help you with the cycling.



The Fluval flex is definitely the more pricey option but for the additional $20, you get a tank that has 4 gallon of space. I think that is quite the deal. In addition, the Fluval Flex also offers better led lights and has the bow front design which can be appealing to some.

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