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In my experimentation with soda making in the eighties, I discovered that CO~2~ injected water tastes really acidy. Club soda has sodium bicarbonate added to offset the flavor (and cheap ones don't balance it very well) and then Tonic Water (with quinine) leans into the sour.

This is the reason that Coca-cola syrup has to have a crazy amount of sugar, or a high-powered artificial sweetner. The other solution is to make your fizzy water less fizzy, but when you're a teenager who believes fizzy is God, that's preposterous.

PS: Also don't mix citrons (e.g. oranges or lemons) with milk, which will do terrible things as well. We have to do weird industrial / chef magic to make orange or lemon flavored ice cream and such.