Misc. Simple Watercooling Questions

You shuldn't add tap water to to any water cooling facilities.

Tap water contains minerals which will be deposited onto the cooling surface of the cooling block, water pumps ...etc which will reduce the efficiency.

The best is to top up with the original fluid that came with the kit or if that is not available, the next best thing is using distilled water (pure H2O).

Another thing to take note, DO NOT mix cooling fluid from different makers.
Each brand got their own recipe which might caused unwanted chemical reactions when mixed.

Thats my opinon anyway,

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.