Can i use tonic water instead of regular water for marshmallow fondant icing ( please help?)?

My daughter is turning 7, and is having a "whip your hair" willow smith, glow in the dark, rollerskating birthday party...(SMH...Girls!) For her cake she wants it to glow in the party room which will be under a black light. I am making marshmallow fondant. I heard that you can make the cake glow by using Tonic water (like Canada dry, the drink, looked it up, thanks google) Should I just use tonic water to make the fondant, would it ruin it, or make it taste funny. Or just spray it lightly on the cake, anyway Im putting a edible picture on there so it needs a spray of "water" anyway.