You're missing the fact that under a scenario where supply is restricted, increasing prices also discourages those who already supplied themselves from exausting the supply. I mean, if water is 0.10€/bottle then I'll gladly buy the whole supply with my pockey change just to avoid any chance I might feel slightly peckish.

How does price-fixing solve that problem?

However if the same bottle is sold for 1€ or 10€ then I'll think twice before pulling out the bottle, and I might even open a stand to sell part of my provisions for less to undercut the competition.