Well, the salt should be marine aquarium seawater mix. The brand couldn't matter less, though I know some people have strong opinions. But for brackish water fish, it doesn't matter.
The easiest way to do this would be to run the 80 litre at SG 1.010 and then gradually raise the SG each week or two, so that the filter bacteria can adapt. Alternatively, set the 80 litre as a marine tank using some marine filter bacteria from another aquarium. Scats can be adjusted to seawater within a few hours by the "drip" method: that is, put them in a large bucket, half filled with fresh/brackish water, and then add a little seawater, either through a small siphon tube (like airpump tubing) or by adding a cup of water every 5-15 minutes. In theory, you should be able to adapt them instantly, but I wouldn't do that; I'd give them at least 1-2 hours to go from fresh to seawater.
By the way, unless you are keeping invertebrates, running the SG at 1.018 rather than 1.024 is a good idea. It puts a lower metabolic strain on the fish and also allows more oxygen in the water (more salt = less oxygen). Hardy marine fish, like damsels and tangs, are fine at SG 1.018.
Cheers,
Neale
Hi everyone thanks for reading 
Mum has had two Silver Scats in freshwater for five weeks and now wants to transfer them to an 80 litre tank and start them in Saltwater.Don't be concerned anyone reading the 80 litre tank is only temporary.Ok the question she wants to know is how much and also what sort of salt should she introduce to the tank.Any answers would be much appreciated.Thanks for reading 
