This is, indeed, the scariest moment for a pond keeper.  Don't panic.  To save the fish that have survived thus far you need to have a clear head and do the following:

First Aid Steps:

1. Test the Water for Ammonia, Nitrites and pH.
2. Change Out Most of the Water and Don't Forget to Add De-Chlorinator if Necessary

Find the Cause:

Test the water's ammonia, nitrites and pH.  The pH should be between 7.0 and 8.0.  There should be no trace of ammonia or nitrites - that's nitrites with an "i", not nitrates with an "a".

Most the time when all the fish are dying at one time it is a problem with water quality.  If the pH is unusually low and it's early in the morning you may be experiencing a pH crash.  If ammonia or nitrites show a reading of over .5 ppm that is enough to kill the fish, especially if the water is warm.  If it's mid-day in summer and the fish are gasping for air it could be low dissolved oxygen.  If you have just treated the pond with any chemical it could be an overdose or the chemical treatment has zapped all the oxygen out of the water.  If this episode follows a heavy rain it could be something (maybe some sort of weed killer) has been washed into the pond.

There is an outside chance that a virus is killing the fish if all of them are suddenly dying.  Have you added new fish in the last few days?  Did you add fish while the water was cold and the water is just now heating up?  

Choose the ONE statement below that best fits the situation:

• My pH is very low.  I think I had a pH crash.
• I just treated the pond and now the fish are dying.
• I added fish recently and I'm afraid they may have had a disease.
• The surviving fish are gulping for air and hanging out by the waterfall or fountain. 
• Nothing fits.  Take me to the Diagnostic Clinic.

Items You Might Need:

Master Liquid
Drop Test Kit

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