Tap water contains silicates, phosphates, copper, and other possible chemicals which are detrimental to your tank water quality, and the health of your invertebrates (and fish).
Silicates can lead to explosive growth of diatoms (an ugly brown crud). Phosphates can lead to the growth of all types of bad microalgae like hair algae and cyanobacteria. Copper equates to death to most all invertebrates.
Tap water: do not use it unless you have a reverse osmosis device in your house.
Stirring the sand is a bad idea in general. First of all, if you have the reccomended flow rate for a fish only tank (10x) or a reef tank (20-25x), then stirring the sand is not necessary. Secondly, stirring the sand too vigorously can lead to accidental exposure of the lower layers of the sand. This can lead to dieoff of your anaerobic bacteria, which play a most vital role in the denitification process.
Please take the advice of all the people here. All we really want to do is prevent you from making common mistakes.