I think 30-40 tetra of any of the "common" type is pushing it in a 30g tank. If the species is one of the quieter ones (i.e., not active swimmers) such as cardinal tetra then in a 30g I would not have more than 15 roughly. And no other mid-water fish; some substrate fish would be OK, and some surface fish like Carnegiella marthae (9-12). And lots of plants, or alternatively a more authentic habitat with sand, lots of wood btranches, and thick floating plants. Minimal filtration (cardinal do not like flowing water, nor light). Very soft and acidic water is mandatory. You can read more in our profile, click the shaded name.