Hi I bought an Aquaero and my case (http://www.nzxt.com/products/lexa_blackline/) also have three temperature sensors. But I don't know if I should install the external sensors or not..... should I put the sensor between the CPU and the heatsink and put some thermal compound in the middle with the sensor ? or is it too danger and I ma destroy the CPU, because I won't have a perfect attach between heatsink and cpu ?

First of all, do not install a sensor between the cpu and heatsink...ever...for any reason. You can use the sensors to monitor ambient air, hdd temps, and so forth just by taping the sensor in an appropriate position. Ambient air temp: sensor goes at bottom front area of the case. HDD: sensor at top center of the hdd. and so forth. Anything else, like a cpu, gpu, or motherboard chipset is, at best, difficult to position and should not be done by a "normal user". Skip