I've noticed that when I dissolve a soluble tablet in plain tap water the tablets dissolves quite quickly, however when I dissolve it in sparkling water it takes far longer. Any ideas why? Could it be due to the extra dissolved CO2 in the water that hinders dissolution?

It could be that the bubbles in the liquid settle on the tablet? Thus there is less surface area of the tablet actually in contact with water. That coupled with your suggestion that the water is diluted by the disolved gasses sounds reasonable. This is just a guess however and I do not actually know.

There may also be an effect due to pH. Sparkling water is full of carbonic acid, so you should use a weak acid as a control rather than water if you don't want pH to have an effect, or a substance not affected by pH. The surface area explanation is also a good one.

Well sparkling water contains a lot of things that normal water doesn't so I wouldn't really expect to dissolve in the same rate. And I think Mr Skeptic is right, carbonic acid (although a weak acid) does affect the pH rate and this probably might be what you're looking for.

No one has considered:

 

Are the temperatures of the two "waters" identical? imp

No one has considered:

 

Are the temperatures of the two "waters" identical? imp

Don't think so. There DIY experiments are usually done in room-conditions so I don't think there might have been a temperature change that could possibly affect.