The Fluval Flex is a modern-looking aquarium with a funky shape and large filtration capacity.

At 41x39x39cm it holds 57 litres/12.5 gallons, which is about the same volume as a standard 60x30x30cm, or two-foot fish tank. It's 19cm shorter, however, so this needs to be taken into account when selecting the fish species that you are going to put into it.

The fish that we recommend are based on maximum adult size and activity. We recommend a maximum adult fish size of 5cm/2” for the Flex, so that could include Dwarf gourami, Rams, and Corydoras catfish, but nothing larger. Fish that need a lot of room to swim up and down the length of the tank, like Tiger barbs, should be avoided. They need to be kept in groups too, so a shoal would use up all of the tank’s stocking capacity. 

We’ve compiled five possible fish communities below, comprising a mix of fish that grow from 2cm in length to about 5cm. Note tetras need to be kept in groups, and these recommendations are based on a mature Fluval Flex 57l which has been running for six weeks or more, with ammonia and nitrite levels at zero. 

We recommend a maximum of 20 small fish in the Fluval Flex 57l. 

Golden rules of fish stocking

All aquariums should be stocked slowly, with regular water tests, and the Fluval Flex is no different. Never buy unknown fish on impulse and always research the maximum size and requirements of every fish you want to add to the tank. Test water before you make a purchase to check that it is ok, and start with the hardiest fish on your list, building up stocks over weeks and months and adding the most sensitive species last.

Tell the store assistant what tank you have, how long it has been set up, your water test results, what fish you want, and the fish you currently have. If Ammonia and/or nitrite are present, no fish should be added to the tank that week.  

Colourful fish

  • 5 x Guppies
  • 6 x Neon tetras
  • 3 x Albino Corydoras
  • 2 x Dwarf gourami
  • 3 x Platies

Or

Nano fish

  • 5 x Pygmy rasbora
  • 5 x Sparkling gourami
  • 5 x Corydoras hasbrosus (dwarf species)
  • 3 x Otocinclus
  • 2 x Scarlet badis

Or

South American fish

  • 6 x Cardinal tetras
  • 2 x Rams
  • 3 x Corydoras panda
  • 5 x Pencilfish
  • 4 x Otocinclus 

Or

Temperate/unheated fish

  • 5 x Zebra danio
  • 5 x Glowlight danio
  • 5 x White Cloud Mountain minnow
  • 3 x Variatus platies

Or

Community tank

  • 3 x Cherry barbs
  • 5 x Harlequin rasbora
  • 3 x Peppered corydoras
  • 3 x Neon dwarf rainbowfish
  • 5 x Glowlight tetra