The genus is probably _Gambusia_ or maybe _Heterandria_ (both are called
Mosquito fish in different areas of the US). They are in the same
family (Poeciliidae) as the Guppy (_Poecilia reticulata_), the Platy,
Swordtail, etc. The family is live-bearing and the majority of the fish
are voracious eaters of Mosquito larvae. Because they bear live fry,
and reproduce quickly, they are very useful at lowering Mosquito
populations and have even been planted in pools in areas where they
previously did not inhabit. BTW, the females can store sperm in their
bodies for a long time and it's not uncommon for baby fish to appear
from 'no where'.
JMS