The fish could be feeding on insect lavra or they could be starved for oxygen.
Two entirely different situations. How many gallons do you think the pond is
and how many fish do you have in?

If oxygen is the problem - can you put in a fountain? There are ones that run
from a solar panel if electricity is a problem. (Go to froogle.com to find
them.) There are air pumps that add more oxygen to the water.

Do you know if the city adds chloramines, you can call and ask. They don't gas
out like chlorine does. A treatment is needed.

Ponds with an assortment of plants and very few fish don't have to be filtered.
But garden ponds are rarely understocked. You can test for ammonia and see if
the plants are keeping up with the fish waste.

Anyway with more information we can track down what's going on.

kathy :-)
zone 7, SE WA state