Not sure what you mean by silver tipped sharks but none of the others need salt. Your "shark" could be a Jordan's Catfish, Arius jordani which would most definitely benefit from salt. I'll track down a pic and post it in a minute so you can see.
That fish is actually a brackish to marine species which can tolerate FW. I've never seen one kept in FW bigger than 6", but I've seen them raised in SW reach well over a foot. That particular fish should be salted 24/7.
