Hi Wsp. Gunnie is right - you must have tested for nitrate. Nitrite, even when very low, can kill your fish. Nitrate, however, can go up even to 40 and you fish may be ok. But it's not good if nitrate is this high anyway - so it's best to keep it way below 40, of course zero would be perfect.
You asked if you should buy bottled water in a grocery store for water changes. Well, when I had only a 10 gallon tank - that's where I was taking my water from. The fish did very well in that water. And pH was neutral all the time. However, we're not talking about 10 gallons but 55 gallons here! If you would have no problem buying so much water, then I don't think there would be anything wrong with it.
Now I have a total of 40 gallons (2 tanks - one 10 and the other 30) and I simply don't find it convenient to be buying some 20 gallons of water every week. Too much hassle. So I use my tap water and dechlorinate it. However, I am lucky because my tap water is also neutral. So the choice is really up to you - if you'd be willing to buy so many gallons and if it wouldn't be a problem to you.
If you decide to buy water that has a much lower pH than the water in your tank - you'll have to
acclimate your fish to a new and much lower pH very slowly. You don't want them to suffer any shocks. And once you start using bottled water, you'll have to continue to do so in order to provide a STABLE pH (remember?) - unless, of course, you find another source of water that will have the same pH as the bottled water.