What is the difference between the Fluval Flex and Fluval Spec?

Shape and lighting. Per Fluval, the Spec is long and narrow (the 5 gallon is 19.5 x 6.5 x 11.5 in.) with a 7000K white LED; the Flex 2.0 is deeper and cube-like (the 15 gallon is 16 x 15 x 15 in.) with RGB plus white LEDs and a remote.

Do Fluval Flex and Spec kits include a heater?

No. Fluval lists heaters as sold separately for the Spec, and the Flex 2.0 listing says heater sold separately.

What filtration do they have?

Both use 3-stage filtration in a rear compartment: mechanical foam, chemical and biological media, per Fluval and the listings.

Fluval Flex vs Fluval Spec: Aquarium Kits Compared

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Quick answerBoth hide a 3-stage filter in a rear compartment and both need a separate heater. The Spec is long, slim and shallow (5 gallon: 19.5 x 6.5 x 11.5 in.) with a bright 7000K white LED, which suits a desk or shelf. The Flex 2.0 is deeper and roomier (9 or 15 gallons) with RGB-plus-white lighting on a remote and a spray bar for gentle flow. For more water volume, the Flex 15 is the bigger tank.

Here's how they compare from Fluval's spec pages and the listings. We haven't tested them.

Side by side

	Spec 5 gal	Flex 2.0, 9 gal	Flex 2.0, 15 gal
Dimensions	19.5 x 6.5 x 11.5 in.	14 x 13 x 13 in.	16 x 15 x 15 in.
Pump output	55-80 US GPH	94 US GPH	127 US GPH
Lighting	7000K white LED	RGB + white, 6500-7000K, remote	RGB + white, 6500-7000K, remote
Filtration	3-stage: foam, carbon, BioMax	3-stage: foam, Poly-Carb, BioMax (listing)	3-stage: foam, Poly-Carb, BioMax (listing)
Flow options	Multi-directional dual outputs	Dual nozzles + spray bar	Dual nozzles + spray bar
Heater	Sold separately	Sold separately	Sold separately

From Fluval's spec pages unless marked "listing." Fluval also sells a 2.6 gallon Spec (formerly Spec III).

Which suits what

Spec: a narrow footprint for desks and shelves, with a strong white light.
Flex 2.0: more water and depth, color-adjustable lighting, and a spray bar Fluval describes as "gentle, even water diffusion for fish requiring less current."

Spec 5

Fluval Spec 5 Gallon Aquarium Kit

5-gallon etched glass tank with aluminum trim, per the listing
7000K LED
3-stage rear filtration: foam, carbon, BioMax
Heater not included (sold separately, per Fluval)

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Spec 2.6

Fluval Spec III 2.6 Gallon Aquarium Kit

2.6-gallon etched glass tank, per the listing
7000K LED
3-stage rear filtration

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Flex 2.0, 9 gal

Fluval Flex 2.0 Aquarium Kit, 9 Gallon

RGB + white LED (6500-7000K) with remote, per the listing
3-stage rear filtration: foam, Poly-Carb, BioMax
Spray bar option for gentle flow
Heater sold separately

Check price on Amazon

Flex 2.0, 15 gal

Fluval Flex 2.0 Aquarium Kit, 15 Gallon

RGB + white LED (6500-7000K) with remote, per the listing
3-stage rear filtration: foam, Poly-Carb, BioMax
Dual output nozzles plus spray bar option
Heater sold separately

Check price on Amazon

Add a heater

For tropical fish, both need one. Fluval's M50 is rated up to 15 gallons and the M100 up to 30, per Fluval. See aquarium heaters.

Fluval M50

Fluval M 50W Heater

For tanks up to 15 gallons, per the listing
Borosilicate glass; reflective finish
Adjustable dial

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Other options in aquarium starter kits.

FAQ

Main difference?

Spec is long and slim; Flex is deeper with RGB lighting.

Heater included?

No, for both.

Filtration?

3-stage rear compartment in both.

Fluval Flex 2.0 vs Fluval Spec

Side by side