There is A LOT to know about them.
They are peculiar fishes, need a very specific tank, with a wide beach where they'll spend much of their time, and brackish water. They are hardy, though. The african species,
Periophtalmus barbarus, is widely traded, but is also the biggest and the most aggressive. Try getting the asian species, a bit more peaceful.
Links:
http

/www.themudskipper.org/
Neale Monks' pages on brackish water describes breafly some of the species: http

/homepage.mac.com/nmonks/Projects/brackishfaq.html
Much more in his book: http

/www.tfhpublications.com/fish/brackish-water-fishes.htm
Also search this same forum, mudskippers are very popular here.
Cheers