Since it's been there for a while, I would expect it's reached a steady state in terms of bacterial cycling. I would say that either you have incoming cloudy water as helenchild suggests or you've got a stagnation-induced oxygenation problem that's preventing the natural cycle of bacteria and algae from clearing the water. Either way, you've got to improve the health of the body of water to deal with it. Around here the farmers know that a bit of greenery around the edges and some fish to stir things up keep the water fresh in a retaining pond, and even then they usually have a sprinkler or fountain of some sort to improve surface oxygenation.