It is mandatory to express concentration of salt (or any chemical material to be used on animals for that mater) based on amount in weight per volume of water or in volume of standard solution of chemical in volume of water (and not per number of animals which is misleading), otherwise we risk over-dozing.
In preventive medication with salt (e.g. when you are trying to quarantine newly introduced fish into your ponds from another farm that you are not sure of), I am aware that 5ppt or 5 parts per thousand is adequate. This means weighing 0.5 kilos of salt into every 100 liters of water or 5 kilos salt into every 1,000 liters of water (whichever is easier to understand).
However, in curative medication (eg when fish are infected with a particular disease outbreak (especially parasitic) and you want to stabilize them with salt before you apply other medications), up to 10 ppt have been used depending on the enormity of the infection. Of course, suspending feeding for a period of up to 3 days has been recommended during the process of salting.