Thanks for the advice folks,
Sadly, I live in a smallish town in Australia where there are only two pet shops - not a lot of choice. The tank has been cycled. I've been checking
ammonia levels since I got the fish and did daily partial changes for a couple of days just to be on the safe/paranoid side. It all seems to be okay, though in fairness, for them to get inland where I am, they probably have a fairly stressful journey, so I'm not too surprised if a few don't survive the extra stress of a move to a new tank.
I may get another couple of serpae, though would welcome any suggestions as to other fish that might be safe to join these two?