no need to add salt..and i agree to wait it out and just go with carbon and water changes and see if it helps ... keep us posted.
Just a quick update.........
it has been a few weeks now and the fish are fine. They two males still flick as well as one of the females, though it is only once a day or so, it has not spread to anyone else and they fish that flick remain healthy and active. I am now putting it down to behavioural as if it was anything bad (ich/flukes etc) i think they would have died by now, or at least appear very ill. it must be either because they are in a smaller tank (they will be moved to a larger 63litre tank soon, currently fishless cycling) or because its a mating thing lol........
I am hoping they all remain well, and guess the flicking may be the odd thing I see from time to time.
I have been checking their water daily, and it remains constant at ph 8.0-8.2, ammonia 0, nitrites 0 and nitrates 20 (same as tap water). they now get a waterchange of 25% once a week. This seems to keep them happy.