Why is my garden pond fish water green and cloudy?

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You have algae bloom. I use a product called Algae Fix from Foster's and Smith. Works well.

check your aerator

If you get a lot of plant in there it will solve the problem.

http://www.apifishcare.co.uk/help-detail.php?id=54

I don't think it will kill your snails, since it doesn't hurt fish in the pond, BUT to be on the safe side, call Foster's & Smith and ask or go online to their website, find it and read the label. Also, I find that I don't get algae bloom anymore, since almost the entire surface area is covered in plant life, like lily pads and parrot's feather. Also, if you fertilize your pond plants, that PROMOTES algae bloom, too. Hope this helps.