If you have fungus gnats, that is an indicator that the soil in the pot has been or is too wet. It may be the plant already had this problem when you bought it, that's hard to know, but do you empty out that outer pot 30 minutes after watering so the plant is not left sitting in water? If not, start doing so. More information on fungus gnats and how to deal with them here
Note that bottled water is not always a good thing to use for houseplants - the sodium level can be quite high in some brands, and that is not a good thing for plants. Rain or spring water (preferably collected yourself from a natural source, or a bottled 'natural spring water' with a low sodium level) are best, but tapwater is also fine for plants, so long as it is not artificially softened.
In terms of watering, only water when the surface of the soil feels dry to the touch, but not so dry the plant is wilting or the soil has shrunk from the sides of pot. Water well, allowing excess to drain away freely from the base, empty the outer pot after 30 minutes, and again 30 minutes later if more has collected.