Fish Deaths

Firstly Water quality is good for Nitrate, Nitrite,PH, Ammonia etc.
A few weeks ago my fish all went very shy and silly. They hid at the
bottom of the pond, were very timid and when fed would flick at there
food and dart away.
Unfortunately the goldfish and shubunkins started dieing first. When
netting the fish out, they looked totally healthy, had good colouring
and no sign of any disease. Then Smaller fish died and the final straw
came when one of my large orfe was at the surface not very well. We
drained the pond and caught the remaining fish which were just Orfe and
Rudd left.
Out of some of the deaths in the pond, some fish had their head and
their tails missing. (morbid I know) We hopefully have managed to save
about half the fish.
What has possibly caused this disaster as we do not just want to refill
with water and send the remaining fish back to their grave. Also after
draining could not see any predators but plenty of Frogs and toads.
Please help any advice would be welcome.
Graham