From the symptoms you describe it would appear that the fish are suffering from a severe bacterial infection.
Before recommending a treatment I suggest you have the water tested for levels of ammonia, nitrites and nitrates as water quality issues are usually the root cause for fish disease in a pond. Test results should indicate ammonia and nitrites=-0- and nitrates 20ppm or lower. Different results would indicate a water quality problem that can be rectified by a series of 15% partial water changes accompanied by a decrease in the amount of food being fed.
After doing the water tests and fixing any problems that might exist you should then begin to treat the fish. Start off by adding some pond salt to the water. Any brand will do. Pond salt will help ward off secondary infections and serves as a general tonic. When administering, just follow the directions on the product lable for the dosage amount.
Once the salt has been added begin to treat the product with a medicine called
TetraPond Pond Fish Treatment, click on link below for a view of the product.
TetraPond Pond Fish Treatment is an excellent pond medicine designed to treat bacterial problems amongst a variety of other ailments. Follow the directions on the product label for usage/dosage instructions.
Best wishes and please let me know if you have any questions.
http://www.drsfostersmith.com/product/prod_display.cfm?c=5163+5304+5408&pcatid=5408
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