A summary explanation may help. This is a basic concept that we must all understand, and it need not be over complicated, so my explanation will be very basic and leave out some detail but it is the general concept you need to understand.
GH is the general hardness of water, which is primarily the amount of dissolved calcium and magnesium in the water. GH does not change much if at all in an aquarium (for simplicity I'll leave out how to do this, that is involved) so the GH of your source (tap) water is what you have to work with. Fish are impacted by GH more than any thing else when it comes to water, except or equally temperature. Generally, there are fish species that function best in soft or very soft water, and species that function best in moderately hard to hard water. Some species manage well in the middle.
GH is measured in several different units, but degrees (dH or dGH) and parts per million (ppm) are the commonly used hobby units. The ppm is the same as mg/l which many water authorities use. You can convert back and forth with the number 17.9, multiplying dGH by 17.9 to get the equivalent ppm, or dividing ppm by 17.9 to get the relevant dGH. I prefer dGH because the numbers are smaller and for me more easily remembered.
KH is the carbonate hardness also called Alkalinity. Fish are affected by this, but much less significantly generally speaking. KH is usually close to GH, so the higher the GH the higher the KH, again generally. I personally do not worry about KH but it is useful to know that it acts like a buffer to maintain the pH and prevent fluctuations. This is critical if one starts adjusting the pH, because the KH can kick in and the pH will return to what it was, depending upon the KH level. This is seriously debilitating for fish, fluctuating pH.
The pH has to do with ions in the water. A pH of 7.0 is neutral, pure water like RO or distilled. Water is a strong solvent, so it readily picks up minerals, organics, CO2, etc that it passes over/through. A pH below 7 is on the acidic side, while a pH above 7 is on the basic side. Generally, a higher GH will mean a more basic pH, and a low GH will usually have a more acidic pH. Fish that prefer soft water (low or no GH) tend to also prefer acidic pH, and fish preferring harder water tend to prefer a more basic pH, so the two go together nicely.
Mollies like all livebearers as an example need mineral in the water because their physiology is designed to use this calcium and magnesium, and if this is not in the water they will not obtain sufficient and this causes serious internal problems and leads to death. Opposite, soft water fish have a physiology that does not need or use these minerals in the water, so when soft water fish are forced to live in harder water they are absorbing these minerals unnecessarily (to explain how would take a book) from the water and this causes internal problems and leads to death.
Adjusting parameters I will not get into as I said, but you would need to deal with the GH/KH and the pH would sort of follow. You have water with a GH of 150 ppm (divide by 17.9 for the equivalent 8 dGH) which is moderately soft. There are a lot of fish that will be fine in this water, but not mollies. There are fish with differing requirements (needing water like you now have) in your post #1 and some of these would not be suitable. As a second tank just for the mollies is not possible, it would be best to return the mollies or otherwise re-home them.
