Check the website of your water authority, it likely has data there and may include GH. The KH (Alkalinity) is also worth knowing. And their pH value (this can change, but GH and KH are less likely to unless targeted. But--this assumes you use the tap water.
You mention in post 1 using RO. If this is the case, then presumably the GH/KH are zero, and the pH around 7 (neutral) unless you mix R/O with some tap.
Phosphates...in R/O these should be zero. Phosphates will occur through fish foods, but hopefully no where else. To your questions.
Salt as meaning common salt, sodium chloride, should
never be added to any freshwater fish aquarium on a "regular" basis, only as a medication for specific disease if salt is the best option. This would be temporary, and most freshwater fish can manage with this. Livebearers do not need sodium chloride, they need mineral salts, especially calcium and magnesium (these make up the GH). While mollies can deal with salt (sodium chloride) better than say soft water species, it is certainly not necessary for good health. Salt
is detrimental to all freshwater fish to varying degrees. My article on Wet Web Media explains this:
The salt issue leads to your question on parameters.
GH is crucial to fish. The pH is less so but important, depending. The KH is not as directly impacting, but it factors in when considering the pH fluctuations.
The GH ranges in the chart are way off the mark. So much so as I would forget the site completely. GH is the level of dissolved "hard" minerals, primarily calcium and magnesium, and these are crucial for fish well-being. There are fish (freshwater only being discussed) that need these minerals in the water because they cannot get sufficient from food. There are other fish that do not need these minerals and can suffer if they are high. It has to do with evolution of each distinct species in a very individual habitat. The fish for instance that live in the Amazon rainforest have evolved to function in very soft water; livebearers as an opposite example have evolved in water containing these minerals. While nothing is cut and dried, there are limits. All of this is part of the individual genetic blueprint of each species of freshwater fish.
Water is continually entering fish via osmosis through the cells (how fish "drink"). Substances dissolved in the water and that are able to diffuse across the cell membranes (minderals can and do) enter the fish's bloodstream and internal organs. The kidneys filter out "toxins" and in the case of soft water fish with calcium it blocks the kidneys and the fish dies. This may be long-term, and it weakens the fish so the fish may succumb to some other issue and the calcium not really be detected as the actual issue. Hard water fish have the opposite problem, getting the minerals they require.
If you have RO water, or very soft to soft water, you are not going to have healthy mollies, or other livebearers. Guppies are a bit of an anomaly but we'll needn't get bogged down further at this point. The tetras, rasboras, gourami, and most any fish from South America, SE Asia, Central Africa will thrive in this water. By contrast, some of these fish may survive but certainly not thrive in water that is hard.
There are other issues to consider, but the above is significant at this stage. I heartily endorse Seriously Fish as a reference for sound advice.
www.seriouslyfish.com
