I am adding Seachem Alkaline Buffer to raise the ph, gh, and kh. (And once in a while, baking soda). But as I mentioned before, my livebearers were super healthy and active. They didn't appear to have anything wrong with them unless ph, gh, and kh affects them internally and is not noticeable in behavior.
What gh level am I looking for?
To your last question, Colin responded. I will just add that these are the minimum levels, and higher is better. The reason is how this affects these fish, which goes to your comment in the preceding paragraph of this post.
Unlike marine (sea) water which is basically the same across the globe in terms of GH, KH and pH, freshwater is uniquely different. Water is the most solvent liquid on this planet, which means that it will easily and readily assimilate substances it comes into contact with, such as organics, minerals in rock, toxins, etc. As condensed pure water falls as rain or snow, it begins to do this, and especially when it reaches the ground. Thus, freshwater is unique according to the geographical area, because of the substances it has taken up.
Freshwater fish have evolved over thousands of years to function in very specific water parameters. Fish such as livebearers have evolved in water that is moderately hard with a basic (above 7.0) pH. Their physiology is geared to this water, and they depend upon the dissolved minerals (primarily calcium and magnesium) to properly function. Fish continually take in water through every cell and at the gills; every substance dissolved in that water is transported into the fish, into the bloodstream and thus to the internal organs. They need the calcium and magnesium which they assimilate out of that water, expelling the waste as urine. When the water they are in does not have sufficient minerals, their physiology must work harder to deal with this, and this can only last so long. There are usually no external signs whatsoever, until the fish will be seen to be lethargic and then dies.
Along the way, this extra "work" caused by the lack of minerals is weaken the fish more and more, making it susceptible to other problems, health issues, etc. It will no longer have the ability to fight off issues that it would ordinarily manage.
With that understanding, you have two options. Stay with your source water and select soft water fish species, or adjust the source water for livebearers. The former is easier and safer because you can use your tap water as it is, which makes water changes much easier. Preparing water means doing this outside the aquarium, and then adding the adjusted water at water changes. That is more work, and it can make emergency water changes (they will occur) very difficult. The water entering the tank must be stable and suitable to the fish.
