I can't agree that blanket weed (a filamentous algae) could be caused by low nutrients. In fact, like most algae, it's growth is promoted by high nutrient levels in the water. In nature, we always find it in areas where the water has been enriched with nutrients, often run-off from farm fields or input from treated sewage where the phosphates have not been stripped, and where vascular plant growth is low. High phosphate is often the cause, something which aquarists generally don't seem to test for. In garden ponds, it usually occurs when people feed their fish, i.e. add nutrients, which inevitably raises nutrient levels via the subsequent fish poo, and do no water changes. Aquarium plants may be competing poorly with the algae, but if you add fertiliser, make sure it is in the form of tabs to the substrate, not to the water which will make things worse.
