Bottled water varies, a lot of it is spring water and like the OP found out can actually have significant nitrates and such. The rest is usually filtered tap water that is then bottled. It is rarely RO water. RO generally isn't sold as drinking water since it is completely stripped of minerals, elements, and natural electrolytes. Water does not like to be pure. Life does not survive in pure water, thats why we remineralize or mix RO water with tap water. Some people do drink RO water for better or worse, but it has the tendency of pulling minerals out of your body, especially calcium. I'm sure they sell products to add to RO water to make it 'super healthy' drinking water. RO water in general tastes like crap IMO, water is tasteless and you store it in plastic so then it tastes like plastic. Typically the more 'stuff' in water the better it tastes. My current tap water isn't the best but it tastes okay. My folks well water with almost double TDS and 25ppm nitrates is great water and my plants loved it too.
Lilybelle - look online for a local water report as usually this is public information. Either link the site here or copy and paste whatever info they offer.
Your low GH and high pH make me wonder if your water suppliers are using corrosion prevention methods, since naturally soft water will corrode water pipes long term its not uncommon for them to jack the pH up to prevent this. It would depend if KH is also medium to low. If it is then the abnormal pH isn't as concerning and not that important with the fish.