questions: asked about fish mortality for oxygen depletion. 
 answers: immediately change the water to @30% or give aeration using aerators.
immediately stop the application of fertilizers and feeds.
if there is no provision for replenishing the water or there is no chance for artificial aeration and if the water level is above 1m: use potassium permanganate @ 250gm/ha and agitate the surface with bamboo.
oxy more  tablets can used to enhance the d.o  levels in the pond water (20 tablet/bigha or 200 gm/bigha).