got ur 2nd hand thermaltake bigwater already???
how much did u buy it for??
u mean theres water in between the arcylic and the block other than the water passage?

try tightening the 4 screw on the arcylic and see if the washer(the black rubber) is placed properly or not .....

wut do u mean??
u wanna pour out the water but dont wanna pour out the coolant???
its impossible to do that as the water and coolant is mixed....

if u wanna take out water, u really have to pour it out...
but its gonna be hard to pour out all water if u didnt take off any tubing unless u disconnect the tubing at the pump intake then it will be easier to pour out...

my suggestion is this:

if u wanna disconnect tubing and change water, then just disconnect it and pour it out the plug back the tubing and refill....

if u dont wanna disconnect the tubing but wanna change water, u can just pour out the water stored in the pump, refill it to its full, run it for a while, then pour the water out again and repeat the same thing several time, then most water will be changed.... after that just refill it and add in some coolant....