Will a salt block kill weeds in pond?

Salt will kill plants and can contaminate soil, groundwater, ponds and plants nearby, not just on your property, including trees if there's too much in the soil. Use sparingly if at all. So use as a last resort.

However salt isn't very effective or long-lasting at killing aquatic weeds and plants in a body of water like a pond. It can change the water chemistry that's needed by other wildlife who also call the pond their home