Fluval Flex 9 Gallon (34L) Review

Last Updated on 2024-07-02

There are so many great nano tanks on the market that your betta can live in. It can be hard to know what’s the best one for you. In this Fluval Flex 9 Gallon review, you’re going to find out everything you need to know such as what it comes with, the benefits and disadvantages and who it’s for!

Fluval Flex 9 Gallon Review – Is It Worth It?

If you’re in a hurry, here’s a table you can use to get a basic understanding of how good the Fluval Flex 9 Gallon is!

ProductFluval Flex 9 Gallon
Size9 Gallons
HeaterSold Separately
Imagefluval flex 9 black

What Comes With The Fluval Flex?

When you purchase this you’re not just getting a tank, you’re getting most of the things you need to start an aquarium. While there are some parts that you’re going to need to buy separately, the extra expense isn’t going to be high. Here’s exactly what you’re going to get:

  • A 3 stage filter that comes with (oversized) mechanical, biological and chemical filtration media. Thee filtration types you need to keep your tank hospitable for bettas.
  • The tank itself that measures 16 x 13 x 13 inches and is 9 gallons or 34 liters in size.
  • 7500k LED lights with a variety of different settings you can choose from.

If you’re looking at these technical details, and you’re not sure what I’m talking about then don’t worry, this review is going to break everything down and make it easy to understand what you’re purchasing.

First of all, the tank.

The Tank

As you can tell by the name the Fluval flex isn’t like other aquariums. It has a bow front which means the front of the tank is coming out slightly, giving it a unique look and feel. And the fact this happens means you’re getting extra swimming space for your betta.

Another great part of the tank is the hidden compartment. In the compartment, you’re going to have plenty of room to store a heater, filter and additional filter media. Parts of the tank also have nooks carved into them so you can safely run wires through them. This gives the front of the tank a very clean look and helps keep it uncluttered.

And lastly, because of the square shape, it means you can put it where you couldn’t put rectangle tanks. I keep mine on my desktop, but people have also put it in cupboards and other areas of the house as well!

However, there is a downside to the tank as well, although it’s not huge. When you keep the lid on a lot of condensation builds up on the top, so when you open it if you’re not careful it’s going to drip everywhere.

To stop this happening you have to pull it forwards slightly as you open it, and then the tank catches all the water. And remember, this problem isn’t limited to the Fluval flex, any tank with a lid is going to suffer from the same thing.


  • Curved front is unique and adds more depth to your tank.
  • The tank itself has little notches that you can put your wires through, which makes it very tidy.
  • A hidden compartment allows you to store away your filter, heater, and any extra filter additions out of the main part of your tank.


  • Water vapor builds up on the bottom of the lid so you have to open it carefully.
Fluval Flex 9 Aquarium Kit – Fish Tank for Fish & Plants – Comes with LED Lights, Filtration System & More – 36″ x 18″ x 18″ – 34 L, 9 Gal. – Black
  • Stylish Fluval Aquarium: Contemporary design with curved front and honeycomb wrap that conceals water line and filter compartment sides

The Filter

Next up is the filter. And for an aquarium kit, it really exceeds expectations. Not only does it come with an oversized mechanical filter, which means more debris is going to to be caught in it, but it also comes with Fluval’s biomax media (which harbors beneficial bacteria) and activated carbon (which removed chemicals).

Because they’re also Fluval’s own brand you know they’re going to be effective and trustworthy. And when you need to change them you can pick them up cheap off Amazon!

Another great benefit of the filter is the amount of space you have to add extra media. There’s plenty of room in the back for you to put water polishers and ammonia removers. This is especially helpful in a smaller tank where ammonia can build up quickly.

And lastly, the filter is nice and quiet, as long as you pull the wires through the part of the tank they’re designated for you can sleep easily with it in your room.

However, it can be hard to clean the hidden compartment where the filter goes. While you won’t need to do it often, you should purchase an algae scrubber to get to the bottom.

People have also said that the current produced by the filter is too strong and it pushes their betta around. While I didn’t notice this there are a few ways you can fix it.

First of all, you can add Fluval’s pre-filter sponge onto the nozzles which will drastically reduce flow rate. If you don’t fancy doing that then you can use a sponge filter to block the intake grills and slow the water down as well. And lastly, if you angle the nozzles out of the water (which I’ve done) it slows the water dramatically.


  • It comes with an oversized mechanical filter which means more debris is going to be caught.
  • Also comes with Fluval’s biomax media and activated carbon which help harbor beneficial bacteria and remove chemicals in the water.
  • The filter is quiet, and if you angle the nozzles out of the water you also get a nice trickle effect.


  • It can be hard to clean where the filter is by hand, so you may need to buy a scrubber.
  • Some people complain that the flow is too strong, however, if you angle the nozzles against the sides of the tank it isn’t a problem.

The Lights

The lights on this tank come with a lot of great benefits and you won’t need to upgrade them anytime soon. One of the best things about this light is the adjustable setting. If the white light is too bright or too dim you can change them.

This comes in handy if you see your betta flaring. Instead of having to turn them off you can just dim them until he can’t see his reflection anymore. And as well as this they also change color. You can choose from a variety of different colors on the color chart or go for a nice moonlight blue.

They also have a “lightning mode” but I wouldn’t recommend leaving it on too long or you may scare your betta.

The lights are also strong enough to support plant life. I currently have some anubias, java moss and marimo moss balls in my tank and they’re all growing fantastically.

The only real downside with these lights is the fact that sometimes they can be too bright. If you have no other light source in your room then the white light often causes a reflection in the inside of the tank, which makes some bettas flare. However, this can be overcome by making sure the light outside your tank is brighter than the light inside your tank.


  • Lights come in a variety of different colors from the color chart including white and moonlight blue. It also has a lightning mode.
  • You can change how bright or dim the white light is.
  • Bright enough to grow plants.


  • If they’re too bright they can cause a reflection inside the tank. You can fix this by lowering how bright they are.
Fluval Flex 9 Aquarium Kit – Fish Tank for Fish & Plants – Comes with LED Lights, Filtration System & More – 36″ x 18″ x 18″ – 34 L, 9 Gal. – Black
  • Stylish Fluval Aquarium: Contemporary design with curved front and honeycomb wrap that conceals water line and filter compartment sides

Setting Up The Fluval Flex 9 Gallon

Setting up the Fluval Flex 9 Gallon isn’t difficult, and it should only take you about half an hour.

The first thing, before anything else is to make sure there aren’t any leaks. To do this you should fill it with water outside or in your bath and check.

To empty the tank use a gravel vacuum, or a cup to remove all the water until it’s empty again.

Place the tank where you want it, add gravel, plants, decorations, your filter, and heater before filling it with water.

Once it’s filled with water add the water conditioner, plug everything in and then turn it all on!

All that’s left after that is to wait for it to cycle and then you’re good to add any fish you want to it!

Stocking The Fluval Flex 9

When you’re stocking the Fluval flex 9 you don’t want to add too many fish. While it can house more than one, you should stick to about 4-5 fish (depending on the fish) with the addition of shrimp and snails.

If you plan on housing a betta, then ideally you’re going to want your betta and 1 or 2 bottom dwellers, or shrimp. This will help stop your betta from feeling like his territory is being taken over.

Who Is This Tank For?

This tank is great for anybody, however, there are some people who it would suit more than others. If you’re any of the following then you’re going to love this tank!

  • A first-time betta owner who wants a good beginner tank.
  • An experienced betta owner who wants to have a planted nano tank.
  • Somebody who wants to upgrade their tank size but still save space in their home.

If this sounds like you then you shouldn’t hesitate to buy it!

Amazon Customer Reviews

There’s nothing better than knowing that other people liked this tank as well. In fact, here are a few of the reviews on Amazon from other people who purchased the tank and love it!

  • I changed from a large tank to this and have never looked back great tank”
  • Great little tank that has a small foot print while still containing a good volume of water.”
  • Lovely tank, am very pleased with it. Very easy set up and love the fact nothing is inside the tank, and all filter, heating etc is at the back.”

What Else Will You Need?

While this tank has almost everything you need to get started there are some things that are missing. If you’re interested in what else you need to get with this tank then here they are:

  • A Heater – This is the only other necessity that you’ll need. I recommend getting the Fluval M 50-Watt Submersible Heater. It’s made for the Fluval flex and fits perfectly in the hidden compartment
  • Substrate – You should also consider getting substrate for your tank. Here are the best gravel substrates, but if you wanted a planted tank then aquarium soil is probably better
  • Plants & Ornaments – Lastly you’re going to need to decorate your tank with plants and ornaments. It’s up to you what you pick, but here are some of the best plants for betta tanks.

Fluval Flex 9 Gallon Review Recap

If you’re looking for a great new tank for your betta then this is a great one. I hope this Fluval flex 9-gallon review has taught you everything you want to know about this tank.

It has a lot of advantages such as a hidden compartment, great filtration and lighting, and a unique look, as well as some disadvantages. But all in all, it’s a great little tank.

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13 thoughts on “Fluval Flex 9 Gallon (34L) Review”

  1. Hi mate,
    Really good reviews for that tank??.
    I’ve just finish to cycle my fluval 9gallons tank and wondering what is the go for cleaning the mechanical filter and the actual pump too?
    How often should I do it and can I turn the pump off??
    How often should I replace the activated carbon and media?
    I’m thinking about removing 5liters of water every 2 weeks so I don’t have to do it every week??

    • Hi Jim,
      I’m glad you like the Fluval 9 Gallon it is a great tank!

      To clean the mechanical filter sponge, you just need to take it out when you’re changing your tank water and give it a good swish around. It normally keeps for a few months before you need to change it.

      I always turn everything on my tank off when cleaning, just to avoid getting anything electrical wet and to stop the heater from breaking.

      Ideally, you should replace the activated carbon every 2-4 weeks, however, chemical filtration isn’t necessary in a lot of cases, so you can normally get away with changing it every couple of months.

      When changing the tank water I remove about 1/5 – 1/4 of the tank every week or couple of weeks.

      I hope this helped!

  2. Hey mate,

    Thank you for those great tips!
    Do you recommend to wash the mechanical filter with the aquarium water I guess??

    • Hey Jim,

      You should wash the mechanical filter in the old water you’ve taken from the tank. If you wash it in normal water, you risk killing off the good bacteria in your filter, and in extreme cases, could damage the bacteria colony and fish in your tank.

      Hope this helped!

  3. I have the fluval flex 9 gallon tank and bought the fluval m heater. Doesn’t fit, can’t close the lid…not fully submersible. Great heater overall, just wish it fit so the lid could close ☹️

    • That’s strange, I have the same filter and tank and it fits perfectly, are you sure the filter isn’t getting in the way?

  4. Also I can’t find the black netted bags that were given with my fluval tank when I first bought it charcoal and Biomax anywhere I only see the small one which is originally for the Spec tank

  5. I bought the bagged charcoal and biomax on Amazon and they fit my Fluval tank filter compartment perfectly. They are much cheaper on Amazon than at the fish store.

  6. I’m the proud owner of a flu all flex. I’m cycling it at the moment, the flu all liquid says use 25ml per 40 litres so I used 20ml and on day two and three will use 7ml had to take a guess as the bottle doesn’t specify. I intend to keep a Siamese fighter and a Bristlenose only as I don’t want to overstock it. I hope this sounds about right to you. Your comments would be useful.

    • Hi, Bristlenose plecos need about 20-30 gallons to thrive, so a 9 gallon tank is going to be quite small for them. If they’re a juvenile it’s fine, however, as they get bigger you’ll have to upgrade your tank.

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