Are water changes a must during antibiotic treatment if ammonia registers zero? Or can I perform massive water change at the end of 7-10 days treatment?
You should follow the instructions on the bottle of whatever product you are using. Some meds degrade faster than others in the water. Always follow the instructions in order to not end up with a concentration that is too high during treatment.
At the end of the treatment period you can do however large of a WC you would like.