Hello I use to work in a garden that had a duck weed problem ,we set a filtration system up using a small empty lemonade bottle,this was fastened to the end of the pump pipe a hole an inch diameter was cut in the side towards the bottom of the bottle the bottle floated we found it dwew enough water in and duck weed for the pump to cope with it the pump out flow was set so that it kept the water circulating past the hole in the bottle,the duck weed couldnt get out and ended up in the filtration tank which was then cleaned out weekly or when the water stopped flowing ,theres also the duck method encourage a duck to eat it,if you can get Algon path cleaner that can be used to clear blanket weed but before using it contact the manufacturer with the pond dimensions for the exact amount to use ,as for an herbicide you need a licence to use it on water Round up Bi aqua is the one you need PA1 chemical handling PA6 knapsack sprayer and a water spray licence when I applied for the course the it was £495+vat +the water certificate at £475+vat the mixing rate for Roundup Bi Aqua is 1 litre of round up to 1 million litres of water that has to be brought down to the pond size you have,you also have to check if theres any outlets into a water course eg ditch or river jezza