Over abundant goldfish(GF) are notorious for making the water turbid in the visibility of 12" - 16" or less. Once GF are over crowded and as adults (8"-12"),,, IMO no predator will ever be able to get them back to low density where the water has clarity of 1.5 to 2 meters or more. If it were mine, I would eradicate all the fish and start over or you will fight overcrowded GF with lots of frustration for many years.