The diet soda and regular soda differ in the amount of solids (sugar/sugar substitute, soft drink ingredients, etc.) that are dissolved in the liquid. The amount of dissolved solids in a liquid will affect the liquids density. An object will become buoyant (float) when it is less dense than the liquid surrounding it. Buoyancy is simply an upward acting force caused by the fluid. Archimedes was the first person to dissect this phenomenon and stated what is now known as Archimedes’ Principle “Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.”
Note that one packet of an artificial sweetener is approximately equal in sweetness to about three packets of sugar and the artificial sweetener weighs less. The regular soda has more solids (soft drink ingredients and sugar) dissolved in the liquid than the diet soda so it is denser than the water surrounding it in the aquarium causing it to sink, and the diet soda has less solids dissolved (soft drink ingredients and artificial sweetener) so it is less dense and floats.
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