Your advice is horrible, and most of you aren't even aware of the single best source of accurate Leopard Gecko information. So let me enlighten you with this video from someone who actually knows what he's talking about. You can thank me later...
FYI, tap water is not isotonic. Its hypotonic just like distilled water. Isotonic water would be VERY bad to give a gecko. Distilled water and tap water are both hypotonic and fine to give to geckos.