Question: query regarding water pollution.
Answer: 1.immediately stop application of feeding and fertilizers.
2.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. 
increase oxygen level-
1.oxy more tablets (20 tablet per bigha or 200 gram per bigha )
2.oxy plus– tab (fish ponds : 600 gm per acre).