Fluval Flex Aquarium Kit - 123L - Black

The Fluval Flex Aquarium Kit is the largest Flex, 123 litres of curved-front glass with an Aquasky LED controlled from a phone and the filtration hidden in the rear compartment.

Key Features

• Vertically bent front glass rather than a flat pane, which is what gives the Flex its curved look
• Rear filtration compartment hides the pump and media behind the honeycomb print, so there is no hardware in the display
• Remote-controlled LED lighting
• Designated heater channel in the rear compartment (heater sold separately)
• Whisper-quiet circulation pump on rubberised feet
• 123 litres (32.5 US gallons)
• Aquasky LED operated from a mobile device, with custom colours and dynamic effects
• Programmable 24-hour light cycle, which matters more than the colour effects — fish and plants both want a consistent day length
• Available in black or white
• Made by Fluval, the aquatics brand of Rolf C. Hagen, which has been building aquarium filters since 1975

Setting It Up

• Put it where it will live before you add water. A full aquarium cannot be moved, and glass on an uneven surface cracks
• Check what the furniture underneath can take. Water weighs a kilo a litre before you add glass, gravel and rock
• A new tank has no bacteria in it. Cycling — running the filter for several weeks until it processes ammonia and nitrite to zero — has to happen before fish go in, or they sit in their own waste while the bacteria catch up
• Plant it and let it settle for a few weeks before the first fish. A tank that has been running a month is a far kinder place to arrive in
• Add fish a few at a time, not all at once

Living With It

• Set the timer to about 8 hours of light a day and leave it. Longer than that grows algae, not plants
• Rear compartment holds the media and the pump; maintenance happens there
• Never wash biological media in tap water and never replace it all at once. Chlorine kills the bacteria living on it, and changing everything together restarts the cycle
• Keep the rear chamber topped up so the pump does not draw air
• Unplug anything electrical before putting your hands in the tank

Good To Know

• Stocking is the thing beginners get wrong. Ask what the fish grows into, not what it is in the shop — most tropicals sold at 2 cm do not stay that size
• Nothing in this kit heats the water unless stated. Tropical fish need a heater; a heater is sized at roughly 1 watt per litre
• A liquid test kit for ammonia, nitrite and nitrate tells you more about a tank than anything you can see
• Most tanks want 20–30% of the water changed weekly, with the replacement dechlorinated and matched for temperature
• 123 L filled weighs roughly 150 kg with substrate. It needs the matching cabinet or a stand rated for the weight — an ordinary sideboard is not enough
• No heater included, and a tank this size needs one for tropical fish
• The colour-changing LED is fun for an evening and irrelevant to the fish. The timer is the feature that matters