The cuda is likely isalinae as it has double black line. The bottom line looks faded out.
Nastasus has one black line. A. minimus is even rarer also has a faint single black stripe. The difference in these two are scale counts and gill rakers. Likely too small to count on small fish.
There isn’t much info out there on them unfortunately. Their behavior is similar to other pike characins.Been googling not much is on these or marble gars do you know of places i can read more?
That thing is the only fish in this thread that's not a tetra. That's a false gar, Xenentodon cancila, from Asia. It inhabits much the same type of habitat as the true gars, but is not related to the characins.