I want to make my own water cooling system and i did some reaserch,
So is it better to reassemble my rig in the water tank or to assemble it outside of the tank and carefully lower it into the tank.
WTF are you talking about!?!?!???? What you ate talking about is NOT water cooling. You are talking about OIL cooling.
For water cooling see: http://www.tomshardware.com/forum/253958-29-watercooling-guide
Educate your self first.