Is it really worth $20 to buy a 500 ml bottle of fluid? Isn't there something equal, but less expensive?
I just realized the tubes running to my external radiator (they're transparent neon green colored) are exposed to sunlight during some hours of the day while I'm at work... does that count as "exposure to direct sunlight" since the sunlight has to go through the tubes?
Lastly, I had so many leaks initially (my fault!) that I had to add some tap water to the greenish fluid that came with my kit... did that compromise my cooling capability?
I agree, don't use tap water. Beyond the issue of minerals is the one of metals in the water, iron or other metals can oxidize and coat the surface of your copper/aluminum cooling solutions which will also reduce efficiency.
Stick to distilled water and add something to reduce corrosion and bacteria growth, there is another thread which discusses what type to add. If you don't want to buy distilled water you might (I don't recommend this but it's cheap) run tap water through something like a Brita filter (to remove solids) then boil it (to remove most bacteria).
In reality, $20 is not really that much compared to what you paid for the water cooling system itself and compared to what can happen if you screw up.