The salt in itself is probably not cause for concern. The problem arises in the additives in many water softener salts. If you are using Morton's in the BLUE bag, you have no cause for concern, but if you are using Morton's in the YELLOW bag you are adding other chemicals to the pond's water column that may have a toxic effect at very low levels (micrograms/Liter). The addition of ANY non-naturally occurring compounds (chemicals) will alter the natural balance of water quality to a lesser or greater degree. This is exactly why the EPA has established fresh water quality guidelines, to aid us in protecting the ecological balance of our waterways. Choosing not to follow thee guidelines, or basic common sense for that matter, is inviting disastrous consequences.