In a pond forum I read of a study done by a pond plant seller (Bonnie's Plants I think) who had done a study with pond plants and salt. They found that 0.2% was lethal to floaters, like duckweed and water lettuce and hyacinths, and 0.3% was the limit for most other pond plants. Now, many of the pond plants are also used as submerged plants, to some degree, and the 0.3% was pretty much across the board of platns tested, so this 0.3% might be considered a limit. That would be 2 and 3 teaspoons per gallon according to my notes on salt in ponds, but this should be checked out with other sources. It's been a year since I read that and this info on those tests is from memory, not notes.
Now, I don't know if any other salts that we may be adding to the pond in the form of fertilizers need to be considered. But even so 2 to 3 teaspoons per gallon is a lot of salt.
You might try to find Bonnie's Plants online and see if that is the location of that testing.