Question
QUESTION: I have a watergarden with goldfish and koi. Recently I noticed that the fish were sluggish and lay at the bottom of the pond - some have died. I have a 75 gal(approx) pond with 2 carbon filters both with aeration fountains. I changed 1/2 of the water, tested for salt levels,ammonia,nitrite,PH,chlorine and everything tested fine, but still the fish have not improved. Found evidence of dead bird but unable to find bird in pond. Could this be my problem?

ANSWER: I can't answer this question completely without some more troubleshooting.

Here are a few theories I have, let me know if you think any of them may be causing your situation:

1) It could be ammonia.  Some ammonia tests only have a shelf life of 4-6 months.

2) Test for nitrate.  The symptoms may be indicative of too much nitrate.

3) Parasites could be to blame.  A sign of parasites is raised scales, between 1-3 scales raised per parasite.  The koi may also try to scrape the side of their bodies against rocks, etc. if there are parasites.

4) If it rained recently, and runoff entered the pond, there could be something toxic in the water, such as a fertilizer.

5) How many fish do you have?  It could possibly be part of the problem.

Did the water change help the condition of your fish?  One of the first things a fish will do if stressed is "clamp" its fins, or hold them tightly to their sides (unless the problem is Oxygen deprivation).  Make a note of things that help.

And don't worry about the bird.  As long as it doesn't decompose in the water, it's not anything to worry about.

You can either respond to this answer or create a new question.
We should be able to get to the bottom of this.

---------- FOLLOW-UP ----------

QUESTION: I have 10 fish in the pond, the same number since the pond was built 10 years age and have had no problem until now. What is the best way to rid the pond of parasites or other bacteria?

Answer
You really do have to diagnose before you treat.  Seek help from someone in your area with experience in the area and a microscope.

Each potential problem has it's very own very unique solution.  It's bad to "guess and check" with antibiotics and other strong medicines.

ALSO, VERY IMPORTANT:

MY ABILITY TO DIAGNOSE IS BASED ON YOUR ABILITY TO CLEARLY, ACCURATELY DESCRIBE SYMPTOMS.  THE MORE TIME AND EFFORT YOU PUT INTO YOUR QUESTION, THE MORE I CAN HELP YOU HELP YOURSELF.