You can keep them in freshwater.. Most LFS not speeking for all.. But most that i have come across throw them in one of there regular tanks...
So better to aclimate to what ever tank if you haven't asked the weight of the water they are holding the fish in

.. MIne were in fresh at the lfs and so i didn't change anything..
They are said to reach 6".. Mine just cleared 2"... Had em for a few months got them at 1.5...
They require excellent water qulaity and are best suited in single specie tanks...
In some hobbiest cases they can be as hard to keep alive a small arrow.... But that is just my 2 cents..