You should use a coolant with ethylene glycol in it, The same stuff that is used in car radiators. Doing this stops the insides of the spindle from corroding and damaging it. The ethylene glycol / water mix should be similar to what is put in a car radiator.
Edit, if you are meaning the liquid to cool the cutters, you can use a water soluble cutting fluid mixed with water at a very high water to cutting fluid ratio. Another option is to use methylated spirits, but sometimes this can damage the plastics in the mister equipment if it is a low grade of meths. I personally would use the methylated spirits as it leaves no residue on the aluminium. You can also use plain water when cutting plastics to increase cutter life. Be aware that methylated spirits should not be used on acrylics as it can cause cracking when the acrylic ages.