Water wetter has multiple purposes.
1-reduce the surface tension, so that the water makes better contact with the walls of the block.
2-helps prevent galvanic corrosion, a phenomenon by which 2 different metals are in electrical contact to each other, typically with water.
3-helps prevent bacterial growth.
So the first question that you want to ask yourself is: is it possible for my rig to experience galvanic corrosion? You have to look at all the metallic components that are in contact with the coolant, including the barbs. If you have Copper as well as Aluminium, then yes, you are in a situation where galvanic corrosion will occur. (If there's anything else, let us know, and we'll take a closer look.)
The surface tension reducer, also known as a surfactant, in commercially available soaps (like laundry detergent) can be substituted for many other things. I'll let everyone else suggest something, but I'll propose laundry detergent, even though I've never tried it.
The bacterial factor is one that's hard to measure, and even observe. It usually just creeps up on you. In short, there are bacterias in all forms of water, no matter what you do to it. Over time, these bacterias multiply, to a point where they start accumulating on the surfces of just about everything, and start altering the performance of the rig.
Some people believe that distilled water alone will cure the problem, when in fact, it only delays it. Some people have suggested alternatives like bleach, methyl blue, phosphoric acid, soap, alcohol, glycol, etc... While I'm not validating any of these, what's important to know is that anything added to the water will decrease the performance of the rig, so whatever is added, needs to be in a small quantity.
Personally, I am going to try windshield wiper fluid: a 1:2 or 1:1 mixture of water and methanol, and I plan to keep my rig all copper.
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