Fish pond

We made a fish pond in the garden this year after 2 weeks added fish and all but 3 fish died? in the last 2 weeks we seem to have gained 25 or more small fish where did they come from? but today we have gone out and 20 at least are dead? the big 1 we had 1st is still there but not moving about much why are they all dying? and surely if the babies got going the pond would be ok, we have a good pump on and plenty of plants in there?? help much apreciated