I am asking to check whether I can prepare an antibiotic solution with a defined concentration for the experimental use from those tablets coming with the traditional antibiotic discs, or should I bring other powder-form antibiotics?
Your question is not clear, but I read it the same as Abhijeet Singh that it is probably the desiccant to keep the discs dry. It is not loaded with antibiotics, and you cannot use it to create a solution. Find either a powder or liquid form of an antibiotic. If you have trouble getting a laboratory grade antibiotic, you might try seeing if your aquaculture researchers have an antibiotic solution, and these are sometimes available at aquarium (pet) stores.