Hey Canyon, REX is right! I would call the MDNR, PRONTO!!!
Fill them accurately on what happened, they are the ones you want to talk to, and I'll bet they will be quite interested.
The aquarium trade is pretty loose and you can get practically anything online. Get caught with something illegal in Michigan, and it could be your, well, worst nightmare!
Like REX said, Cover Your Ass.
Being a nice neighbor is all fine and well, and that's what we hope for so we all get along, but we live in the real world. If your neighbor was being thoughtful and considerate, being a nice neighbor, he would not have trespassed on your property and dumped his junk in your pond.
I would get tough and get the Gestapo involved. No need to be nice to people who have already violated you and your property.
