I was dusting out my tower when I noticed that the water cooling reservoir was looking a little low, most noticeable by the fact that condensation drops had appeared on the clear panel. This brings me to my question, what do I refill it with? I understand that it must be a pure water based coolant, but do I get the pure water or the Apex? Also, when refilling, do I need to purge the system of the existing fluid or do I top it off? How would I do this?