I want to start the prefermentation for my ciabatta. What temperature should I shoot for? Should I use my bottled water or tap?
Tap water, unless your domestic supply is obviously too heavily chlorinated.
Water temperature should be sufficient to maintain your overnight ferment in excess of 23*C, but less than 27*C. Only you know the variables particular to your environment [ie room temperature and flour temperature]
Best of luck
ps. overall hydration for ciabatta in this recipe is low. But, see how you get on
Our tap water is highly alkaline and has a strong taste unless filtered, so I use filtered water.
At my mother-in-law's or parent's house, where the tap water is sweet, I use it straight and don't filter it first.
When I made this recipe about a month ago, I just used room-temperature everything. Turned out GREAT, and it was easy. Made me feel like a pro!