Pond fish are dying approx one a month?

I can't understand what is going on, the water is great so the aquatics shop who tested it say, no disease or parasites are visible, but each month a fish starts to stay at the bottom of the pond and becomes sick doesn't seem to feed anymore until it just lays on its side and hides behind anything until it is so ill there is no way it will recover and this is usually around a month from start to finish from the fish starting to be ill until it has hardly any life in it so i put it out of its misery. I did have 3 young koi fish last june from a aquatics shop then over the winter they formed what i believed to be carp pox. As the weather warmed up it disappeared on two of the three fish but one still has one on its fin, but that is the only thing i saw wrong with the fish, but none of the others have any of these symptoms in the 6 years of having this pond. There is a type of algae that covers the pond liner not string algae but a slimy kind which breaks away from the liner and floats on the top which i scoop of each day and the tadpoles in my wildlife pond love to eat. So If you have any idea what it is i would be grateful for your help. Many thanks