Freshwater Aquarium not doing so well...?

I have a freshwater aquarium and bought 2 Neon Tetras, 1 Cardinal Tetra, 1 Swordtail, and 3 Sunset Mickey Mouse Platys. My Neon Tetras died, And I don't know where 2 Sunset Mickeys are, and I'm also missing my Cardinal Tetra. Please Help!