questions: query regarding improved oxygen level. 
 answers: immediately stop the application of feed and fertilizers.apply lime (quick lime)-10-13 kg/bigha/month, use 4-5 ppm (mg/lit) potash to disinfected the pond water. use toximar 5kg/bigha  to absorbs toxic gases from the pond.
increase oxygen level- oxymore tablet (20 tablet per bigha or 200 gram per bigha), oxyplus tablets (600 gm per acre).