Spark, first, Fish are not "legally" considered livestock unless your state defines them as such, so it is apples and oranges. Plus as a matter of reality, you can not restrain a fish fry that is 1/4" long and 1/16" or smaller in diameter AND leave excess water flow unrestricted. If a dam fails, that is probably a liability or neglect issue as a pond should have been designed in anticipation of the event...again, legally speaking.
As for chemicals, again, if you are downstream, you take on a presumption that chemicals will enter your property...it is YOUR responsibility if you feel it will harm your property...Again, just because you want something special or out the "ordinary", it is incumbent upon YOU to protect it...not your up stream neighbor....in fact, wanting something special, REQUIRES you to take special precautions to protect it. Building a pond without total control over the "Natural" flow of land lays all the risks and responsibilities for it at your feet.
As for chemicals, again, if the chemicals are legal, and legally applied to use in pond A, and they harm pond B. The owner of pond B accepted all risk of what may flow in....
