If the fish are doing well I would not use the algaecide. It could have the effect of killing off so much plankton that you may have DO problems. I would watch it and with cooler weather it should dissipate. If you decide to use the algaecide I would dilute it way down and spray only about 5-10% 0f the surface area with a light dose. The yard fert. is most likely adding to the problem.