The Black Skirt (sometimes called Black Widow) Tetra, Gymnocorymbus ternetzi, is prone to fin nip if kept in too small a group, so you want as many as you can manage in the tank. I would suggest a minimum of seven; I tend to prefer seeing odd numbers in smallish groups, for no reason here other than aesthetic appearance. But a 20g long (30 inches/75 cm length) can hold this. And a group of 8-9 neon tetra. Be regular (weekly) with a partial water change of 1/2 the tank and you should be fine. Live plants will help too.