Stocking For A 37 Litre Tank

That depends on how hard your water is.

The easy answer is a single betta, male or female.

Otherwise -
Very soft water - a shoal of any of the Boraras species  http://www.seriouslyfish.com/species/search/boraras

Soft to middling water - a shoal of ember tetras or a shaol of Microdevario kubotai http://www.seriouslyfish.com/species/microdevario-kubotai/  (often sold as neon green rasboras or green tetras)

Hard water - a group of endlers livebearers


These small shoaling fish do better with a group of 10+; endlers, being non-shoaling, can get away with less than that.

Correction on tank size after actually measuring it myself and not going by what it says on the box.

W.48 H .32  D.25 cms

PH 7.5  DGH 10

I am guessing the only suitable fish would be a betta

Thank's sue, One other option could be pygmy cory's according to aquadvisor or do you think they need more space if they do then i think i am looking at a betta, but would have loved ember tetras or neon green rasboras but it looks like they need a minimum  53cm long tank.