I usually add enough for the whole tank. It kind of depends on what kind of water conditioner you're using. The one I use, API Stress Coat, also neutralizes ammonia so any extra that goes in is used to take care of excess ammonia. Most other water conditioners don't neutralize ammonia though. If you're not sure how much you want to add, I would suggest adding enough to treat the new water you are putting in plus maybe a drop or two extra to be safe. And just in case you weren't aware, add the water conditioner to the new water BEFORE you pour it into the tank. Good luck.