Question: asking about removal of aquatic weed, itching of skin insi9de the water and fish death due to oxygen problem 
Answer: remove 80 percent of the aquatic weeds by using net and leave the rest in a corner of the pond by using a bamboo float on the water as a separator so that the weeds dont spread  on the whole pond surface.
apply lime 5 kg for half bigha water mixing the lime with suitable amount of water and mix well and keep it for one night. apply the solution on the next sunny day.
after 1 gay apply oxymore tablet @ 10 numbers for your pond, apply the tablet directly into thenpond water.
as the fishes are already inside the pond recommendation for chemical for weed removal is not recommended
 for now please dont apply fertilizer,  however the recommended dose for urea is 3.5 kg for 12 bigha and ssp is 2.7 kg per bigha (please dont apply fort few month