Blacking out when standing up from sitting position is a very common thing during a water fast. You have the remedy for it. You can also take a deep breath before standing up.
Filtered Rain water is fine, no need to add anything to it to make it alkaline. I posted a message about water yesterday, which has a lot of info on water.
Do you know the pH of your rain water? How are you collecting the rain water? That also makes a big difference.
For fasting I would suggest to buy a few gallons of distilled water.