Hello,
Ropefish absolutely do not need salt. Adding salt stresses the fish. Ropefish are primary freshwater fish, just like bichirs. In the wild, yes, they do sometimes occur in very slightly brackish water, but only in the same way as things like kribs and spiny eels: because they are hardy, and not because they are adapted to brackish water.
What has complicated this is that this species is common in the Niger Delta, which some aquarists have interpreted as meaning they are from the Niger estuary. A delta is simply the large, triangular region where a river opens out across a coastal plain; it may be brackish, but only towards the edge. The part the ropefish live in is essentially freshwater, with lots of plants as well as things like synodontis catfish, characins, cichlids, and so on.
Ideally, keep ropefish in the same water conditions as you'd keep any other African freshwater fish. Also, they are very sociable, and I'd recommend 3 as the minimum number.
Cheers,
Neale