About five years ago I put in an outdoor 800lt pond with a pump.  I had a bio filter etc etc and my goldfish still died.  All I have now is the pump to circulate the water.  The longest lasting has been this last batch.  I bought four.  They survived for two years.  A couple of months ago two died.  Now today another one.  I have one left.  They always die of the same symptoms.  It looks like something is attacking them - they lose their scales and their fins look like they are being eaten.  It seems to always happen in the frog mating season - I am always fishing mating frogs out the pond from around November to February (live in South Africa).  If it is a disease why dont they all die at once?
Anyone got any ideas?

I agree with cheri, a pond needs a filter and also needs to be cycled.
Have you been cycling the pond before adding fish?

It also may be, that the mating frogs are causing issues. Could it be that during the mating season, frogs are latching on to the goldfish? Or it is possible the frogs are protecting their young if there is any frog spawn about?

I don't have a huge amount of experience, but having looked into keeping a pond myself with frogs and goldfish, these issues come to mind.