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Sodium thiosulphate, purchased in bulk.  ('Cause it's way too pricey at the pet shops for what it is).  The question to me is,how much is too much?

And Chris - you're so right, it doesn't take much to nuke the fish.  I was topping off my pond earlier this summer with the garden hose, and got distracted.  Lost a few fish as a result, and that's just tap water concentration of SH or Chloramine.

Here is a tip for smaller backyard ponds. Use a LARGE beach ball and place it in the center of the pond. Throw the tarp over the pond...pull it tight and stake the corners with plastic tent stakes. Use plastic stakes because they have shallow underground wires for the pond pump, filter and lights. Plastic stakes will deflect the wire and not tear into it plus you deminish the rish of getting shocked! 

The Koi/goldfish ponds that I have had to work around all had aquatic plants and lily pads and this "tenting" method has worked very well to protect them.