Check the following possibilities:
Are grass clippings or other debris getting into the pond and decaying?
If you allow any organic material to decay in your pond, the ecosystem is thrown out of balance and fish and plants die. Remove dead or dying water lily pads and flowers. Remove any other dead or dying plants. Of course, remove dead fish immediately. Your pond could easily be crystal clear and the ecosystem could be out of balance. Be aware and check your pond at least once weekly. I also advise changing at least 10% of the water weekly.
Have you or your neighbors sprayed fertilizers, pesticides or insecticides?
With termite problem in New Orleans, most people have a pest control service to kill termites and other bug critters. Although post Katrina, the termites have evacuated, we expect them to return forthwith and spraying will begin anew.
Many folks hire horticultural companies who specialize in spraying the garden for every kind of bug, destructive or beneficial. Not only does this practice kill all the good bugs, it keeps butterflies and hummingbirds from visiting, it also can easily kill your pond.
Has the local government done any spraying nearby lately?
This is a problem here in New Orleans because we have mosquitoes and therefore we have a mosquito control program. They send trucks out to destroy mosquitoes. They swear they will not harm plants or fish, but I am suspect of any chemicals.
Is someone painting or scraping a house or car in the area?
Paint from the house, especially lead paint, can get in your pond and kill everything.
Has cleaning been done on surrounding driveways, decks or walkways and run off into the pond?
I include this only because I was at my wit's end several years ago trying to figure out why I could not keep a client's pond healthy. After months of investigation, I finally asked a housekeeper if she was cleaning the surrounding patio. She said, "Of course, I scrub it and hose it down every day." OK, there was my answer. She stopped and the pond was healthy again.
Did someone feed the fish too much or the wrong thing?
Feeding goldfish is probably the biggest problem pond people have. All of you want to feed those poor fish. Those poor fish need to eat what is good for them; submerged vegetation, algae and mosquito larvae. Please make them work for their room and board. Koi must be fed and live in a pond with filtration.
Did you use a new concrete block to prop a water plant up? Use bricks or aged concrete blocks?
Lime can leach out of the concrete and quickly throw the pH into the stratosphere.
If any of these events have occurred, change the water immediately. Don't forget the dechlor.