Question
I have a a 4000 litre outside pond with a biological filter with many goldfish and 3 koi. th goldfish have been dying mostly one or two at a time over the last 2 to 3 weeks. I have gone through all the normally things replaced the water, removed a plant that was put in pond 2 months ago, treated for internal and external bacterial and fungal infections, tested water ph 7.5 and total salts ok. I have also treated pond for alage water however is crystal clear. I have done a autopsy on one fish and found little all fish that have died show no visual symptoms. However those that die are a little sad a day before they die. Any idea what could be killing my fish? Any clues appreciated.

Answer
It could be a virus (least likely), bacteria, or parasite, but test for ammonia and nitrate to be certain that neither of those are responsible for the problem. Ease off the use of algaecides until the fish are better.  You can try to treat for both bacteria at the same time but make sure to perform a water change as directed by the bottle.  This is due to the strength of the medicines; it is harmful if left in the water for too long.  Some diseases require a very specific medicine to fix.  Certain diagnosis using cell samples under a microscope may save lots of trial and error.