try a uv fitler or a charcoal filter to keep the water clear. You can also get biological filters for sensitive envirnments like a coy pond.

Steve

I just discovered a product that does a great job with this - Fountec. I couldn't find it at Home Depot or Lowes, so I bought it online. My fountain had quite a bit of algae. I applied Fountec and a week later the water is clear. The best part is that it does not kill plants and is safe for birds.

This might not work depending on your design. I used to keep my ponds perfectly clear using old pillows. I had the water pumping and filtering down through that white stuff from a regular pillow.
I could remove it wash out the slime and reverse it. It worked great month after month.