I'd also add that they do best in brackish water, with an SG of around 1.005. They can be found in freshwater in the wild, but live longer and are healthier when kept in brackish water in captivity.
Also, while they can be "nippy", it isn't uncommon for people to keep them with other brackish water fish, such as knight gobies (my figure 8 and knight gobies have been together without problems for 9 months now).