X1 is a premixed coolant. If it is premixed as in 1 Ltr then it needs nothing as it contains every thing you need in the liquid. If it is the 250ml Concentrate version then you only need di water or distilled water. the 250ml concentrate bottle will make 2 ltrs of coolant.
All Mayhems coolants and premixed liquids contain biocide and inhibitors and require nothing extra such as a kill coil. Never mix them with any other biocide such as PT nuke or biocide extreme.