Question: query regarding fish mortality due to oxygen dipletion.
Answer: critical stage(under, severe stress): immediately change the water @ 30% or give aeration using aerators. 
*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:  potassium permanganate @ 250 grams/ 0.1 ha and agitate the surface with bamboo. 
*if the water level is low: reduce the stock to half and stop all feeds and fertilizers till the pond regains its healthy condition.
increase oxygen level-
1.oxy more tablets 20-25 per bigha or 200 gm perbigha.
oxyplus 600 gm per acre.