We have a big problem with blanket weed in our pond and last year we tried Tetra Pond - Pond Balance and it has worked wonders for ours. If in doubt visit your nearest respected koi dealer who will be able to give you the best possible advice as not all the products on the market are effective and some can damage plants in your pond if you have any. If you are not sure where your nearest koi dealer is, buy Koi Carp magazine and they have a huge list of dealers.