Paint Thinning Advice Needed

I find when painting minis, that they get textured very quickly (in the sense of the paint looking like it’s not been thinned down enough even though it has been), and have a feeling it may be due to the water where I live (need a water filter to drink it as it’s too hard, and doesn’t taste right). Would distilled water or a medium be a good idea to get around this? The texture ruins minis that I paint, and I allow time to dry but it still happens. Thank you!