Neon Tetra's need to be in a shoal of at least 6. Also, has your tank been cycled? Or did you just put water in and throw the fish in? Neon's need to be put in a tank that has been established for at least 6 months since they aren't very hardy fish these days. Also what eddie said about the guppy's. The males need to be 2 to 1 to the females.