i have four and no-they are not freshwater. they are always mislabeled as freshwater. they are brackish water. they can be kept in freshwater when they are very young but should be gradually weened to brackish water. most people keep them in fresh water and wonder why they die half a year later.
they prefer a sandy substrate as gravel could cut their delicate stomachs. they also only seem to take frozen bloodworms or tetra delica with bloodworms (it comes in little squeeze packets). the older ones feed at night. the younger ones will eat at just about anytime.
i have 2 babies with bb gobies in a 5 gal and 2 1 1/2 inch ones in a 10gal with guppies.
if you look them up on the web, often a lot of inaccurate information is given. not too much seems to be known about them. they are native to america though and can be found in estuaries in the south. all of the information above is from my personal experience. they are also called hogchokers.
they are great pets if given the right environment.