Well, i dont see why it wouldnt be better than the water your trying to avoid. Most of the time bottled water is the exact same tap water, just ran through large brita-like filters to make the water taste good or different. This is not always the case, but is evident. Look, i know you dont want to, but getting the ro
unit just really is cost effective. If you go to the pet store to buy their water everytime you need to do water changes, your going to wind up exceeding the cost of just purchasing a machine that makes perfectly good water. You can get an RO
unit for 130$ and a GOOD one at that. Try ebay. This will not only save you mass money, but you know exactly whats in the water, unlike if you bought it from some other place, who may add other things in the water, or maybe just dont care and mix it with other things. Who knows. An RO
unit is your best option.
See it this way, you have the tank for years, and everytime you do a water change.. you need water from your pet shop, or wherever you get it from. Eventually in those years, you will have payed alot more money for the water, than if you purchased the RO
unit at a ONE TIME price of 130$ which makes water.. anytime.. anywhere.. when you need it. Idk, choice is yours, but it seemed like a rational decision to me.. so i bought one. lol. Whichever way you go, good luck!