I have a small pond (I don't really know the volume, maybe 400 liters) with goldfish inside, they seem healthy with no apparent problem.
The pond is constantly invaded by micro-algae that cannot be filtered, the water is constantly green and I can see about 5 cm deep from the surface.
From what I've read it doesn't really bother the fish and they actually like it, but I have no easy way to check the health and number of fish in the pond if I can't see below the surface.
I've tried using a product for killing off algae and a UV filter, which worked for a bit, but they seem to have no effect now (my suspicion is that the UV filter might be dirty).
Also, the quantity of algae varies quite a bit based on the season, it once got almost totally clear (strangely without using any special product), but I wasn't able to replicate that result.
I tried also adding some plants, but the fish (and an hailstorm for the floating ones) destroyed them.
If that can help, I live in northern Italy.