Really, fishes (either fingerlings or juveniles) do pass through a lot of stress in the cause of cropping them, transporting and stocking them in the pond where they will grow to table size. The extent of the stress depends on the distance covered in transporting them, the time of the day they were transported and how they were handled during stocking.
Therefore, if you notice that they were stressed in the cause of of all these, giving them anti-stress is necessary. You can as well consult an expert for the right choice of anti-stress.
Meanwhile antibiotics should be given to the fishes only when recommended, it should not be a routine practice otherwise when they are sick and in need of the antibiotics, they might have develop resistance to most of them.
Do you wish to know how drugs can be administered to the fishes? Well commonly, drugs are introduced into the water in the pond. The level of water is reduced to an appreciable depth usually about a foot, the drug is then introduced based on the quantity recommended, you then leave both the treated water and the fishes for some time usually between 30mins and an hour.
After this time, you can then pump in fresh water to add to the water to which drug is added.
Note that reducing the water level of the pond before adding the drug will aid the efficiency of the drug.
As regards to how early fishes respond to treatment, well like any other animal, fish will respond to treatment immediately you are treating the right disease using the right drug. You only have to be sure of the disease you have on ground so that the right drug can be administered which means you need to consult your expert.