Posted: 23/06/2012 at 09:56
What causes blanketweed - a pond which is high in nitrates and phosphates plus sun.
How to prevent blanketweed - expel nitrates, reduce phosphates and sun.
Another possibility is something which is high in nutrients must be leaching into the water.
Pulling it out is the worse thing you can do, unless it is seriously severe, because this releases more spores back into the water, further adding to the problem.
What kind of compost did you use to plant up ? Hopefully it wasn't garden. If it was that's probably the source of this problem.