No, you don't empty half the tank. You siphon/empty 20 or 30% (approximately 1/4 of your tankwater) out of the tank . But before doing that you need to make sure you have a bucket of tapwater with added dechlorinator drops , it should be left at least an hour in the same room as the tank so its the same temperature when you change it. (This is also known as water ager or water conditioner - ie something that removes chlorine and chloramines from the tapwater. If you don't have one you can pick a bottle of it up at petshops or some supermarkets in the pet section)