Help To Identify Fish!

Hello everyone!
 
I have recently agreed to take ownership of my (emigrating) friend's small tropical tank, but unfortunately he is unable to tell me what species the 3 fish in it are (it was a gift from someone else and he has never been that interested in them unfortunately!)
 
I'm entirely new to tropical fish and I can't figure out what species they are- can someone help?
 
Attached is a photo of one of the fish:
 

That's a platy Xiphophorus sp.; a male as it happens.

Welcome to the forum

to TFF, jm!
It's definitely a male platy as fluttermoth said. They aren't exactly tropical they are more temperate which means they need a decent temperature tho not as hot as tropical and not as cool as cold-water. Are they all the same type of fish? If there are any females watch out as they breed very easily!
EDIT: How small is the small tank? They really need to be in a 15G/60L minimum for the swimming space.

You can read more about these fish here:
 
http/www.seriouslyfish.com/species/xiphophorus-variatus/
 
That will give you habitat, size tank, temperature, feeding and lot of other good info.