There is no fish in this pond. It operated smoothly for years with no strange algae until this last winter. It gets early morning sun until 1 pm daily. The pump is 4000gph/hr and it is 800 gallons. I keep this pond very clean and the water tests great. Its like it has an infection. When i drain the water and spray the algae with hydrogen peroxide, the peroxide actually sizzles and bubbles! I have tried nuking it with algaefix but it just comes right back. I have also hit it with sodium percarbonate and hydrogen peroxide but nothing seems to really do the job. It incases all my plants amd basically suffocates them.