kyohti, like many beginners in the hobby, you've fallen for the hype and marketing of many various products on the market. i assume you're a beginner because much of the "information" you're stating is erroneous.
those 'stress coat' substances in those water conditioners are there to recreate that natural balance of microbes and slimes
actually, they're there to take money from your wallet. for example, Stress Coat contains aloe. while aloe is a 'good thing' for burns in humans, it does absolsutely nothing to fish. moreover, the concentration of aloe in there is so insignificant that once diluted in your aquarium there's virtually nothing but parts per billion remaining. moreover, there's nothing in Stress Coat which can recreate the natural balance of microbes and slimes. only the fish can do that.
With bottled water, tap, well, whatever water you use, you're going to want to add a water conditioning solution. You never know what kinds of minerals or chemicals have been added to them. Also, most have an added substance that acts like a fish's natural slime coating and helps to keep down the stress levels of your fish during water changes or tank set ups.
bottled water, tap water ... whatever, has an added substance that acts like a fish's natural slime coating? wow. the manufacturers of bottled water are going to be surprised to hear that. this is absolute poppycock.
water conditioner, which usually also neutralizes minerals and chemicals and has other added benefits.
yuo've made the erroneous assumption that the minerals in plain ol' fresh water are a 'bad thing'. wrong. while some water aquifers may contain irritant heavy metals, you should test the water first and not assume all chemicals/minerals in it are negative.
True that bettas don't really have any specific water needs, but 8.4 is too extreme. Bettas actually prefer a mid to low-range pH and breed best in water treated with blackwater extract,
betta in the hobby today are not wild caught and thus, are not preferential to low range pH found in their native habitat. in fact, generationally, betta fish have not seen the rice fields of cambodia for 20 years now. betta are bred in tanks using whatever water happens to be the norm for the local water supply. it's simple ecomonics.
Again, if he wants to provide a more accurate environment for his fish, despite the fact that bettas are hearty and can acclimate to many different water conditions, why heckle him
i certainly didn't heckle him. i'm trying to help him understand the truths and separate out the myths of the hobby.