Fluval Flex 9 Modification Guide: Filter and Light Upgrades

The Fluval Flex 9 ships as a freshwater desktop tank but its three-chamber rear compartment is almost a ready-made reef sump — which is why it has become the most-modded nano tank in Singapore. This fluval flex 9 modification guide draws on two decades of custom aquarium builds at Gensou Aquascaping in Everton Park. The stock light, stock pump, and stock media are the weakest points. Upgrade those three and a Flex 9 converts into a credible soft-coral and LPS reef at roughly half the price of a purpose-built nano AIO.

Quick Facts

• Tank volume: 34 litres (9 US gallons) stock
• Rear chambers: three compartments — filter, heater/skimmer, return
• Stock light: single 11W LED, inadequate for coral
• Stock pump: Fluval A205, roughly 378 L/h
• Recommended light upgrade: AI Prime 16HD or Kessil A80 Tuna Blue
• Recommended pump upgrade: Sicce Syncra Silent 1.0 or Eheim Compact+ 1000
• Singapore tank price: $180-220, full mod build $600-800 total

Light Upgrade to AI Prime 16HD

The stock LED is fine for low-light plants and nothing more. For soft corals, zoanthids, and mushrooms an AI Prime 16HD mounted on a Prime HD Tank Mount clamps onto the rear glass lip and clears the lid opening. Programme via the myAI app with a standard reef AB+ spectrum at 50-70% peak intensity. A Kessil A80 Tuna Blue is the alternative for those who prefer fixed-spectrum simplicity — it costs less and provides enough PAR for the top two-thirds of the tank.

Chamber 1: Filter and Media

The first chamber holds the stock sponge cartridge and BioMax rings. Replace with filter floss wrapped around a small media bag of Seachem Matrix or Marine Pure spheres. For reef use, add a 100ml bag of RowaPhos in the flow path. Clean the floss weekly during water changes. The chamber is tight — do not overfill or flow through chambers 2 and 3 drops.

Chamber 2: Heater and Skimmer

The middle chamber houses the heater in the stock freshwater layout. For reef conversion this becomes the skimmer chamber. A Tunze 9001 nano skimmer fits with the skimmer body in the chamber and the air intake routed through the lid cutout. Water level in chamber 2 is set by the return pump flow rate, so tune the skimmer after the new pump is installed and running steady for 24 hours.

Chamber 3: Return Pump Upgrade

Swap the stock A205 for a Sicce Syncra Silent 1.0 (950 L/h) or Eheim Compact+ 1000. Both fit the chamber dimensions and deliver the flow a proper skimmer needs. The factory return nozzle is narrow; ream it out with a 10mm drill bit or replace with a locline 3/4-inch fitting for adjustable flow direction. Chamber 3 also accommodates an Autoaqua Smart ATO Micro sensor for automatic top-off.

Powerhead for Flow

The stock return alone provides inadequate flow for coral. Add a Sicce Voyager Nano 2000 or Hydor Koralia Nano 900 on the side glass pointing across the rockwork. Run on a simple timer cycling 10 minutes on, 10 minutes off, or connect to an Apex Nano for wave patterns. Total in-tank flow should sit around 20-30x tank volume per hour including return.

Lid and Evaporation

The stock lid restricts gas exchange and traps heat. For reef use, replace with a cut-to-size sheet of 3mm clear acrylic with ventilation slots, or run open-top with a mesh cover to stop jumpers. Expect 500-800ml per day evaporation in an air-conditioned room, more without AC. An ATO is essentially mandatory on a reef Flex 9.

Common Issues

Microbubbles from the return are the most frequent complaint. Adding a sponge pre-filter on the pump intake and raising chamber 3 water level reduces them substantially. Overheating in Singapore is the other issue — ambient 30°C rooms push a modded Flex 9 to 29°C easily. A small TECO TK500 chiller teed into the return line or a clip-on fan solves this.

Related Reading

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Nano Reef Lighting PAR Requirements