The only viable option is to remove the fish immediately into a separate container with dechlorinated water. If you have a spare pond or aquarium filter then I would start this up as well. Then you will have to physically drain the pond and leave it to soak with some fresh water overnight.
When you are ready to start re-stocking the pond, test the water for pH, ammonia, nitrite and nitrate. Also refrain from feeding the fish for a few days to avoid any potential complications with the water quality.
In the future, make sure that you keep any strong chemicals such as algaecides under lock and key.