Q: I have a 500 gallon fish pond with 25 goldfish. It has green algae in it. How can I control the algae?
A: I think you have too many fish for the pond….and the presence of green algae confirms it. You need to measure/estimate the total length of your fish population.
According to the source below, you can have 25 fish inches if the pond is unfiltered, 50 fish inches if you have a pump/filter combination and 100 inches if you have one of the expensive filter systems.
If you have too many fish, you either need to get rid of a few or upgrade your filter system.