Dying Fish



It is hard to say what killed your pond fish or is making them sick without examining the pond and water in detail.
Sudden fish loss like that can have several causes, among them:
a) a nitrite and/or ammonia spike. Low cost test kits are available to check for this. If this is the problem then all feeding of the fish should be stopped immediately and partial water changes done until the level of these poisons return to normal.
b) some sort of poison washed into the pond or drifted into the pond on a breeze (fertilizer from the lawn, pesticide being sprayed by you or a close neighbor on flowers or trees, etc etc)
c) a sudden deficit in oxygen (pumps fail due to power outages or too many fish for the surface area of the pond).
Water that looks clear and healthy is not necessarily healthy for the fish. A lot of poisons cannot be seen.
At this point testing the water for nitrite and ammonia, doing a partial water change and observing the fish for further signs of stress is the best course of action (please be sure to de-chlorinate any water being added to the pond.