You can buy potassium chloride powder at the supermarket. It is sold as a salt-substitute. I don't know what it would be called in the States, but in Canada and the UK it is called Lo-Salt and No-Salt.
There is a post somewhere here by mouseclick, I think, where he gives a url about a woman who fasted for 92 days but she made a solution of the RDA of calcium, magnesium, potassium and salt in a litre of water and drank it every 3 days. So she was getting a 1/3 of the RDA on the assumption that the body does not need as many nutrients while fasting.
I will try to find the link, or perhaps mouseclick will post it again. I would also just take one multivitamin per day and forget about all the other ones. But I would buy a more expensive multivitamin (not the Walgreens one). You can buy liquid vitamins that contain high amounts of all the vitamins, particularly the B vitamins. Some of them omit Iron, and if this is the case then I would also take liquid iron (Floradix is the best liquid iron supplement).
So in my opinion you should make you own mineral water with mag/cal/pot/salt and take 1 good quality one-a-day vitamin and also liquid iron. I think the RDA for potassium is 1,750 but new reports are suggesting it should be higher (3,500) but I think that would be too harsh while fasting. The body uses less nutrients while fasting.
The experienced water fasters here are against taking supplements while fasting, however yours might be a different case due to the length of your fast and your busy home life etc.
Make sure you post more videos on youtube after your fast because a lot of people are following your progress and it would be a shame if you just disappeared and left people wondering.
~ Loosie ~