Re: Magnesium deficiency on my last fast perhaps due to alkaline water?

Unfortunately I can't confirm anything in terms of not getting into electrolyte troubles. I believe it is very important to have had optimal diet/drinks for a while beforehand, so the body has good reserves of them in good proportions.
I am guessing that the pH 9.5 water you had did not have much Magnesium in it, or the Calcium, Sodium etc were much higher, so it made a disservice during the fast. Epsom Salt baths/feet soaking could be a solution for low Magnesium.
The other thing would be - did your food include magnesium; soils are now depleted from it (and from certain other minerals like Iodine), so the majority of people are sub-clinically deficient. Meaning it does not present overtly as numbness, spasms, constipation, etc, but the body is already functioning under the optimal level and it takes some stress, like fasting, to make it overt.