Ok. Just clean everything..
Get a bucket, a few freezer bags, a hose and pond cleaner.
1) Fill the bucket half full with current pond water.
2) Fill the freezerbags with current pond water.
3) Catch fish and put them in freezerbags and put the bags in the bucket, but not letting the fish escape from the bags. (This keeps them at the same temperature)
4) Syphon out pond and poke the base of the pond at the same time with a blunt stick to free the grime on the bottom.
5) Get out your pond cleaning fluid and folow the instructions. Use a new brillo pad get rid of hard gunk on the lining.
6) Clean the filter sponge in fresh water, rolling it around in your hands and giving it a good squeeze. Also (if your filter is a fluval or one of many other brands) take it apart and clean the little spinny star shaped thing which will probably be gunked up.
7) Fill the pond up again with hose water. (Some people say use water from streams but a hose is good enough).
8) Still with the fish in the bag, put the freezer bags into the pond, keeping the top of the bags above the surface of the water to prevent them from sinking. This alows the fish to adjust to the change in water temperature, even thought they are cold waters. (Use a rock or something to hold the bags at the edge of the pond)
9) After 20 mins, let the fish free into the pond.
DONE!
No more dead fish!
Why do I know so much about fish when I havnt had any for about 5 years?
Sam