If you live in a soft water area, then tap water is OK, but if you live in a hard water like me, I would suggest distilled water, My tap water has chalk in it, so makes the paint look too chalky.
Liquitex slow dri blending fluid is good, however, as the others have said, it does not improve the painting much, I\'m having all sorts of problems trying to use it. I think I will stick to water in future. Its cheaper and works well.
Good luck!