blanket weed grows well if there is excess nutrients in the water to combat this these nutrients would need to be removed by either changing large volumes of water (which can be dangerous to organisms and hard work) or by added more faster growing plants to outcompete the blanket weed for the excess nutrients also a filtration system would aid the break down of ammonia into nitrates which would increase the amount of 'fertiliser' in the water however the water movement would make it harder for the blanket weed to get a good foothold all types of algae hate water movement so moving water is benificial i would
add a filtration pump to aid the nitrogen cycle and increase water movement
clear as much blanketweed by hand as possible
add more fast growing plants
remove any dead or decaying matter
good luck