Our pond water has a cloudy green cast to it.  What is causing this and how do we get rid of it?

What you have there is floating microscopic algae, the inevitable consequence of hungry fish, warm water and sunlight. Your fish eat and from the food produce ammonia, converted first to nitrite and then to nitrate by your biofilter. Nitrate is fertilizer and algae just love it. Not only will the algae cloud your water, turning it to a liquid reminiscent of pea soup, it will adversely impact your fish, especially during hot weather and high water temperature. Algae does produce oxygen, but only during the day. At night, the metabolism changes and algae will deplete your water of dissolved oxygen and severely stress or even smother your fish. You have a couple of options.

The first (and slowest) is to shade your pond and plant lots of pond marginal plants, water lilies, reeds and lotus (if you have the room) to suck up all that nitrate and starve out the algae. A quicker way would be to buy an appropriately-sized UV system and put it into your system after your main biofilter. (UV works best on clean water). This will clean out the algae very quickly, indeed. Older UV systems and the less expensive modern systems need to slow the water flow past the UV source to fully treat the water. This generally means narrowing the diameter of the feed line by as much as an inch. If this is the case with your system, you'll need to rig bypass piping to maintain your flows through your biofilter and into your pond. Newer and high-end UV systems use high-intensity sources and are engineered to maintain normal flows through 2-inch ( or larger!) diameter piping. Make sure you get a UV unit big enough to handle your flow rate, and that your pump can handle the increased pressure head if you get one of the smaller UV units.

(Answer courtesy of Bob Passovoy)