If you've been using regular tap water to do water changes, adding bottled water at this time might not be a good thing to do because the ph might be drastically different, and that could be the final straw for your fish. Your master test kit will also contain a ph test if you get the one from Aquarium Pharmaceuticals.
Your maintenance may be determined at how often your nitrates get above 20. You should do a water change/gravel vac when it gets above that number. Once your tank is cycled and you are not adding fish or changing anything, you will get into a routine and will know how often to do this. Some folks just decide how often to do tank maintenance. I do water changes/gravel vacs once a week. Fish love fresh water, and you can't do too many water changes!
