Hi Guys,
Not many people seem to know that jacks actually spend most of their juvenile stages in freshwater. They stay in freshwater (when available) until they are big enough to move out into the estuary systems, then move offshore to breed.
They look very different when they are really small. Thet are usually a lot darker but have vertical white stripes down them, still got the classic jack shape though. There is a spot up near Mackay that has 100's of jacks up to about 40cm in purely freshwater, trapped in rock pools.
So you shouldn't have any problems with converting them to salt water but just take it slowly as Andrew suggested. Fish like this are classed as euryhaline - maenaing they are able to tolerate either fresh or salt water. There are many species that are like this including barra.
Richard