Water Cooling What Coolant Is Best To Use ???

Computer Type: PC/Desktop
OS: 8.1 / 10
CPU: i7 6950X
Motherboard: ASUS X99A II
Memory: 32GB GSkill TridentZ 3200
Graphics Card: 2 x 1080 FTW
Monitor(s) Displays: 3 x 1080 120hz / 1 x 4k
Screen Resolution: 1080 / 4k
PSU: EVGA SuperNova 1000P2 / EVGA SuperNova 750P2
Case: 2 x Thermaltake Core X9s, stacked
Cooling: custom loop
Hard Drives: 120GB HyperX SSD 2 x 1TB WD Black in RAID 0 512GB Samsung 950 Pro