Hiya! Actually it is not a stupid question at all and comes up quite frequently...The problem with "bottled water" (other then the obvious expense) is that it can very greatly in pH, GH and KH depending on where it was bottled...Different sources can yield very different water and could cause a serious problem in your tank and kill your stock if you had to change brands or the company started tapping another site...
"Distilled" (RO Water) is even worse...It contains absolutley nothing, no disolved minerals and a very low, easily shifed pH...Very few fish require this, none of which you list.
Unless your tap water is from the Love Canal, it is probably absolutely fine for your tank...Check you pH, GH and KH to know your numbers...Very important information to know.
Happy fishkeeping.