Keep it Real.
No need to add anything.
While not needed, go ahead and add a slice of lemon or lime (the real stuff). In our exploration of flavors to add to water, we could not find any that passed our test. Sure, some say "All Natural", but we've found that the labeling is open to interpretation, and, in many cases, does not report on the processes used for flavor extraction. Just like their artificial counterparts, natural flavors are complex chemical formulas created by food companies and a handful of flavor houses around the world. The FDA lets companies call these formulas natural even if they use synthetic solvents or preservatives because it classifies filler ingredients as "incidental additives". When you see "natural flavor" on a label, it's really a black box of secrecy.