Several FA's can have a "protecticve mucilage" and chelated copper such as Cutrine and Captain can struggle to control. If the brown algae in the pic is not dead as often this is the apearence of departing FA, consider adding a labeled rate of a diquat type product like Knockout or Reward with the cheleted copper and add an aquatic surfactant like CideKick or DyneAmic to relieve the tension from your spray droplet and see if this does not work better for you.Install an aeration system that has as much as 3 turns per day depending on the shape and total goals of your pond.With the age of your pond a soaker hose type diffuser may foul easily and if enough lift is not maintained you may prolong your problem by hanging nutrients in the water column making them very available for FA. When duckweed and watermeal or any nutrient consuming plants or algae are eliminated keep in mind the nutrients are still there and the next in line will now flourish.Keep us posted.
