There was a tray of stagnant water with tree leaves in it and my little 18 pound dog drank some of it. He has had mucous diarrhea and small amount of vomiting for a day or so and he will not eat his food or drink.

As difficult as it may be, you need to bag up the diarrhea as best you can and bring it into your vet's office for testing. He might have picked up Giardia or something else. The vet's office is the only place to figure out exactly what is causing this and he/she will know what treatment is required. I hope your little guy recovers quickly.

Ducks and other birds are high risk carriers for parvo, was this a puddle or a body of water? And has your dog been vaccinated and up to date with vaccines?