As Tropheus will confirm, I am a bit fanatical about Diplogramma, I wrote that artical. (i need a trumpet smiley here) I could go on for hours about them.
As already said no salt. Keep in fresh water. The 180x60x60 should be fine. I really do suspect them to be relitively small. I would however recomend giving them as much space as possible, mine is much more active now it is in my 8'
Lots of Channa can be subdivided into smaller groups, Stewarti group, Gachua group etc. Micropeltes and Pleuros are together within a group and the two of these are poles apart in behavour in captivity. In the wild outside of breeding Micros are relatively social with there own kind. It's the confines of space in captivity that increases the impact of their behavour. I strongly believe Diplogramma are part of the same group. Very simaler shape colouration and general behavour to the Micro and Pleuro. Somewhere between the two.
Generally open water swimmers are less terratorial because they have fairly lose terratories. Not known yet but I suspect they become aggresive only when breeding.
Cost wise they are relatively expensive but not excessively, in the UK probably about £200. They are for some reason difficult to get hold of.
Any question please ask.