questions: query regarding decreasing do level in pond water. 
 answers: to increase dissolve oxygen- 1. critical stage(under, severe stress): immediately change the water @ 30% or give aeration using aerators.
2. 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.
3. if the water level is low: reduce the stock to half and stop all feeds and fertilizers till the pond regains its healthy condition.
improving do- oxyzen tablet (8 tablet per bigha), oxy more tablets (20 tablet per bigha or 200 gram per bigha).