bottled water change

Well, what exactly is wrong with your city water? You can get a water quality report from your local water comission and also find out about how they treat the water: do they use chloramines or just chlorine?

I don't think that there are many places in the western world where tap water quality is so bad on a regular basis that you couldn't keep some kind of fish with great success.

If your water folks use chloramines, look into a conditioner that deals with the chloramines and detoxifies the released ammonia. I think that Prime and Amquel do this. There are probably others, but I've never concerned myself with it, no chloramines here.

You should be aiming to change 50% of old tank water per week, that's 5g of your 10g tank if you do it in one shot or 6g in 2 3g removals (33% twice is a 50% change due to the dilution and removal of some of the new water at the second change). I don't know how much your bottled water costs, but I'm willing to wager that it's less than a bottle of top of the line conditioner, or at least over the lifespan of the conditioner it would pay for itself.

Plus when you get a bigger tank this will no longer be econimical or convenient.