Can you give fish antibiotics to your cat?
No - the antibiotics are formulated very differently for fish than they are for cats and you could end up killing your cat.
If you believe your cat needs antibiotic treatment, you should schedule an appointment with your veterinarian for an examination. He/she will provide the proper antibiotics if needed.
You should never give antibiotics to any patient - cat, dog, human - without a prescription. The reason is that antibiotics only work for certain types of bacteria - if you use them for diseases that cannot be treated with that type of antibiotics, the medication will do more harm than good. It will not cure the disease, but the patient can experience unwanted side effects. In addition, misuse of antibiotics contributes to creating resistant bacteria that can cause very problematic diseases. Your vet will tell you what type of antibiotics, if any, your cat needs, and if leftover medication from your dog will do.
The time a cat can take antibiotics, depends on what it is for. If your cat has really bad cat flu it could be on it for over 7 days. A mild dose of cat flu will have a cat on antibiotics for a week at a maximum. Cats have to have antibiotics for more than one reason so the time on it will depend on what ia is for.
Never give ANY human medication to a cat without checking with a vet. Their liver does not process medication like ours does, or even like a dog's. Cats can be made sick or killed by many medications. There are antibiotics a cat can take, but you must check with a vet to ensure it's appropriate for your pet. Also, you should never give antibiotics to a human or animal unless you know that what they have is bacterial. Antibiotics are not effective against viruses. You need a proper diagnosis.