I had a 50 year old pond that was completly full of duck and was silted in. I had the the pond cleaned out and is about 15 feet deep now. This pond was dry for over a year and I figured the duckweed problem would solve itself since i cleaned out all the silt. But this year it finally filled up and the damn duckweed is back. Does water Ph have anything to do with duckweed. Will liming the pond help or will I have to buy the expensive spray and keep spraying it until its gone?