Poorly pond fish!!can anyone help?

ok,so i have a pond in my garden which has coy and goldfish there is quite alot in there i must say,we got a load of fish off someone who was getting rid!we have treated the water constantly but they seem to be dying,theres like one a day now!!!its heartbreaking when yesterday i thort one was dead as it was on its side but wen i went closer it moved a little but ***** stay off his side!!we tried everything but he died this morning :( and there is another one now poorly!there is no physical signs and im going crazy with worry i dont want my fishys to die :( please help