Thinning citadel paints

[Ash Tekka]

Just curious do people use tap water or distilled water to thin their paints before painting(with a brush)?

or is there something that works better?

 

When I thin the foundation paint it appears grainy when sitting in my pallet but I don't see that with the regular paint or the metallics.

 

Also had the issue that when one of my coats of foundation dried it looked like there were little cracks all over it, that went away after the next coat and didn't mess anything up in the long run.