Most diet soda’s use the sweetner found in Equal, which IIRC is Nutrasweet. Yes now some use Splenda or even Asulfate-K (or something like that), but that is sort of recent. For diet tonic water it seems like most use the sweet and low sweetner, which gives it a off taste to me, while around me only one brand uses the Equal sweetner. Why is this?
It tastes better with grapefruit juice 
As someone who’d rather suck a battery than drink tonic water, put me in the “That stuff is sweetened?” camp.