Thank u for your inputs.

We had stopped feeding and fertilization etc, and since we are also draining and adding fresh water there is v little plankton/algae in it currently.
We have commenced feeding but continue to drain.
Somehow it's continuously supersaturated for the past 1-1/2month. The whole column (abt 6ft deep) is saturated.

It's one unit 75 acres, poly carp fish farm, around 63,000 fish initially and after die offs about 59,000.

Water source is seepage water from a river. Before 1963 the river used to flow from here but after the construction of head-works it's flow was diverted.So technically this farm is a portion of the river bed,mainly sandy soil. It had a lot of weeds and floating plants i.e water hyacinth etc.Now after alot of efforts it's clean.Some portion was removed manually and a bit more by using paraquat.