If you start using the pH down you will need to add every day....if you are worried about introducing the fish then add some of your water from the tank into the fish bag over a period of 30 minutes or so while you are acclimatising the fish to the tank temp. When starting up a new tank the pH is the least of your worries. Also pH in a tank changes naturally throughout the day when fish are in it anyway, due to organic waste, oxygen levels, photosynthesis from the plants etc. Also driftwood/bogwood will naturally lower your pH over time too.