Cheers Roy, i knew about the brackish water thing, i'll not be attempting any experiments! What's interesting me is that this has happened within a week of it being moved from a hi tech C02 etc tank to a very low tech tank. The low tech is lightly warmer 25c rather than 22c, but water is the same.
They are clearly much happier in the low tech, more active, out and about all the time - rather than skulking in the shadows.