Water

Now here comes the most widely discussed aspect. There are many schools of thoughts on this. Ideally, one would go for distilled water with a bit of water wetter solution. This is the best as calcium build up will be minimal and bacterial build up is almost eliminated. Too much water wetter will screw up your tubing's as it will cause deterioration along the tubing's wall. Use at most 2 capfuls. Others have been known to use instead coolant to prevent corrosion but personally, it only causes an increase of temps by about 2C and if left too long in the loop, will mess your tubing's and cause sludging (gooey build up on the surface of the loop) which is a pain to clean. If you're still determined to use a water + coolant mixture, I'd say again about 2 capufuls of coolant would suffice. Want the UV effect? then use neon dye instead. But for the best compromise in terms of cost and protection, just use only distilled water, and flush and clean your loops constantly. Those who find getting distilled water a chore, filtered water is a good option but you have be more constant in your servicing intervals.