Hi Dave,
The salt-pan sole is a freshwater (not brackish) species from Australia. It doesn't seem at all regularly traded, despite being featured in a number of aquarium books. I'm sure it can be kept at low salinities, but in the wild it is an honest-to-goodness freshwater species.
At the moment, you are much more likely to encounter things like the hogchoker sole Trinectes maculatus or one of the Asian pan soles Brachirus spp. In the UK, Brachirus orientalis seems especially common, often being sold as Brachirus pan or Brachirus panoides. Both Trinectes maculatus and Brachirus orientalis will do well at mid to high salinities.
To be honest, none of the soles is an "easy" fish. They are nocturnal, and can be difficult to wean onto dead foods. I kept Brachirus orientalis for a few months, but lost it when I had a pH crash, so what I can say is that this species doesn't tolerate pH levels below 6! On the other hand, i used to look after a juvenile coldwater plaice, and it was greedy and hardy, even at room temperatures. Loved trout pellets! So it seems to me that the species offered to aquarists are on the delicate end of the spectrum as far as flatfish go.
If I was planning on keeping a 'freshwater' sole again, I think I'd try and ensure a steady supply of live foods to begin with. Bloodworms and tubifex are ideal for the smaller specimens. Fatten the fish up in a quarantine tank before combining with other fishes: chances are that they haven't eaten for weeks before you buy them. Healthy fish are chunky, while sickly ones look limp and you can almost see the bones.
Cheers,
Neale