Tbh its the anti-caking agents in regular salt that can be a problem, not the salt itself and only because they can degrade to cyanide at levels that can be fatal to small fish. You should also avoid salts with iodine added to them too.
Salt in a freshwater tank has 2 uses normally, in low doses of about 1-2 teaspoons per gallon it can help prevent a type of anaemia caused by nitrite poisoning, but not all fish respond well to this and its better to dilute the nitrite out of the water while a filter recovers. If your fish is heavily covered in a fungal, bacterial or parasitic infection then you can make a stronger salt solution and give a fish five minute salt dip, this is uncomfortable for the fish but often fatal to the problem organisms. If this is unsuccessful you can also use potassium permanganate but that's a bit in depth for this topic.
Can you use the wood? Possibly, depends on a few things and to make it safe takes weeks, but if it looks nice and you want to try then I would do this:
Bake the wood in an oven, this dries it out and kills any nasty little creatures hiding on it that might emerge in your tank and attack your fishes like dragon fly larvae. Also helps kill any fungus, bacteria and parasites that may be living on it. Bleach isn't very good for this since it will soak into the wood and take forever to leach out and remove/neutralise.
Put the wood outside in a tub/bucket and soak it for 6 weeks+. Change the water often or if it rains just let that take care of it. You do this for many reasons like finding out if its a soft or hard wood, soft wood will rot and disintegrate in water and you don't want this happening in your tank. If there is a nasty fungus/bacteria on it then it will sprout and you'll see big black or grey type growths coming out of it. If this happens you'll probably want to bin it as some of these are not to good for fish and can make them poorly too.
Once you're happy the wood is sinking on its own in the bucket/tub and you're happy with the a mount of tannins it is leaching (these aren't normally dangerous and is something called humic acid, tetras will like it, guppies don't really, carbon in a filter will remove this if you don't like it) then go to your local fish shop, buy some live daphnia and add it to the bucket with the wood. If its not too cold they should happily live in the bucket and breed if the water has some food for them, algae or brewers yeast will do - after a couple of weeks of this you can assume the wood is safe and add it to the tank. If the daphnia just die off quickly then the wood is leaching something toxic you can't see meaning you can't use it safely, bin it and get something nice from an aquatic shop instead.
