As has been mentioned, the cardinal tetra is a very soft acidic water fish. They should never be in the same tank as livebearers which are completely opposite in terms of their water parameters. There is no "middle ground" for either in my opinion, so they should not be in the same tank as one (or both) will suffer.
If you are not aware, we have a fish profiles section and many of the regularly-seen freshwater fish are included. Click on the second tab from the left in the blue bar at the top of the screen to go to the profiles section, or in posts if the fish name is shaded it means the profile can be accessed directly by clicking on the name. There is information on suitable water parameters, numbers, tank sizes, compatibility, etc. for the fish in the profiles so you can research fish before acquisition and be sure of suitability.