It takes a "lot" of GreenClean granular to touch it. You probably have it under the floating mats so a dual application would be needed.When you start treating acre feet it isnt practical a lot of times. GC will burn out (oxidize) any plant tissue it comes in contact with. Coverage on submersed targets is the problem.If the conditions that allowed it this season occur next year it will be back. It thrives from the salt water of Tampa Bay to the frigid waters of Lake Erie.Google "lyngbya" and read all about it