[Camdoc]

1. mineral water. Mineral water with gas, or with a lot of bicarbonate, will buffer (cancel) out some of the citric acid, so you would need to use more. You should be able to find some bottled drinking water, no mineralado, and use that.
2. water filter. No, a water filter will not remove minerals. A Reverse Osmosis water filter will. THey are more expensive, though. Should be able to buy distilled water in Spain. Check at a farmacia, they will have it. On the other hand, this stuff will kill anything in tap water, so you could mix it with tap water.
3. citric acid THose deposits around the citric acid container top are not minerals, they are citric acid crystals. Taste them! :-)
4. vinegar--Vinegar does not, and cannot activate yeast. This is a strange myth that goes around the natropathic circles. (Nothing against NDs, who may understand this, but too many laypersons get this confused. Including Jim Humble.)

Vinegar is produced by bacteria breaking down sugar in the fruit juice. Some vinegar, such as wine vinegar, is produced by bacteria breaking down alcohol. This alcohol was produced by yeast, which made it by converting the sugar in the juice. So, in these kinds of vinegar, there may remain yeast byproducts.

People sensitive to yeast often have an allergy to yeast, and will react to their byproducts. So, they may react to the dead yeast, and their byproducts, in vinegars such as wine vinegar. But, the vinegar DOES NOT activate yeast, as many believe. Distilled vinegar (white vinegar) does not have the yeast byproducts, so is fine to use with MMS. Other vinegars are also fine, if you do not have a yeast sensitivity.
With my MD hat on.