I can kind of understand using mineral water if the mineral content is more than your tap water but if you don't know the ph and hardness of your tap water then I can't see how that would be good advice.
I think it is important to know what your tap water is if you use it. Tap water is controlled by the water companies and they are not always truthful about what they do to the water and the main problem I get is the sudden smell of strong chlorine which makes me sick . Other values I measure from time to time stay the same. I know water companies probably wouldn't admit to certain issues but being drinking water they still have to adhere to certain standards I suppose.
I am not sure about mineral water and how good that is or if it is really a good idea to use. It is a natural product they tell you and supposedly better for your health ,so would the values in mineral water always be the same and how would the values in mineral water compare to tap water.
What mineral water brand do you use since they all have different values?
http://www.mineralwaters.org/index.php?func=dispes&parval=all
I did consider using mineral water to add to a shrimp tank for calcium but didn't bother as my tap water has some minerals so not needed.
Technically I suppose it is rainwater so RO then filtered as it passes through rock so minerals added back into it like you would using RO but would you still have TDS like tap water since it hasn't been filtered through a system.
What would be the cost of bottled mineral water compared to just using tap water treated with something such as prime which removes most toxins that urgently need removing.
Do people who bottle mineral water sterilise the water and how, I would have to know more about a specific mineral water before I used it and not just add it because someone from a pet store said it would be a good idea. If you know the person and know they are a very experienced fish keeper and have used that particular product themselves than maybe.
A lot of info is false or untested and gets passed around from site to site and forum to forum and then untrained staff of pet stores read that info and pass it onto newbies or experienced as if they are experts. Then others go on with these methods and rumours become fact.
I think things should be researched thoroughly before doing anything and maybe better to stick to what the majority of experienced fish keepers do until you know the facts better.
I don't trust anyone in pet stores unless maybe it is their store personally but then again even that doesn't necessarily mean you will get good advice since they don't keep the fish for long and almost as soon as fish are imported they are sold so they don't really have to care and are like stated interested in sales. Not all are like that but there are a few and you have to be careful.
I can't see what a pet store would get out of saying mineral water is good for say plecs apart from it would or could make the water unsuitable and your plecs die then you need to go back for more.
You need to keep fish together that have near enough the same requirements if possible and if your tap water is already close to what they need use that and avoid ph up and ph down stuff so it stays the same. You create problems for yourself if you start adding things.
If you can afford RO then use that and add water you have already set to what you want. I find treated tap water fine for keeping fish not breeding and my plants like it.
