Necessary precaution you should bear in mind when you change water. Check the in coming water with the chlorine tester. So you know the concentration of chlorine at this moment. Sometimes water contains no chlorine. Water conditioners you apply vary all the time.
Amount of water you change (25% or 50%) does not matter. You can even change 100% (We always do).
After you have topped up the water, add the water conditioners gradually. Check the water with the chlorine tester every time you add in the water conditioner, until you are satisfied and make sure there is no more chlorine.