Your case is similar to one of my friend's customers when his cardinals were one by one eliminated by his serpae tetras in a 20 gallons alone.:shake: I had to explain that serpaes are quite one of the most vicious tetras you'll ever get, one reason why I loathed serpaes as I had them before and did quite enough damage to make me blast them off from my tank, and they should never be mixed with smaller tetras, not even in a tank less than 30 gallons.
I am inclined to stick warning signs everywhere including the lawn that red-eye, serpaes and black widows/skirts should be banned in tanks less than 30 gallons.:roll: These are a few of the tetras that I loath even if my passion for the characins is still strong. I do not generally prefer vicious fish.:shake:
I would advise that you get another tank and this time place all the red-eye tetras by themselves. If you wish to buy more cardinals, you may do so but I would advise that larger tetras should be avoided. Other tetras you may wish to keep are rummy-nose tetras, neon tetras, diamond tetras, glowlight tetras, lemon tetras, beacon tetras, black phantom tetras and red phantom tetras.