First off, an explanation, as TDS and GH are related but not the same thing. The following comes from an article I wrote a few years back.
Total Dissolved Solids (TDS) is a measure of the combined content of all organic and inorganic substances contained in the water in molecular, ionized or micro-granular (colloidal sol) suspended form. Generally the operational definition is that the solids must be small enough to survive filtration through a sieve the size of two micrometer.
TDS is connected to GH (general hardness) because like GH, TDS includes the calcium, magnesium and other “hard” mineral ions; these ions are what we measure with our GH test kits. But water hardness correctly considered is more than this; both GH and KH can affect hardness and TDS levels; however, the reverse is not necessarily true. Aquarium water can have a high TDS level but a low GH and KH (Jensen, 2009). The TDS for instance also includes sodium (salt) ions, chemical substances, etc. which are not reflected in the GH.
TDS is basically everything dissolved in the water: chlorine, chloramine, ammonia, phosphate, salt, hard minerals (GH), bicarbonates (KH), etc. And almost every substance added to the water will increase TDS: water conditioner, fish foods, plant fertilizers, calcareous substances, medications, water adjustment products, etc.
Fish do not appreciate a high TDS, but their tolerance varies with the species. Soft water fish have a much lower tolerance than hard water fish. But with the latter, it is the mineral content, primarily calcium and magnesium, that is important because the fish's physiology needs these minerals in their environmental water.
There are mineral salts you can buy--and note, "salt" here is not common table salt (sodium chloride) but the salts of minerals like calcium and magnesium--such as those for rift lake cichlids. However, this can get expensive as you need to add these with every water change. When I wanted to keep mollies and even rift lake cichlids many years ago, with my zero GH/KH tap water, I used a substrate of dolomite. I haven't seen this for some time now, but there are aragonite substrates. Just make sure it is not for marine tanks as those will have sodium chloride (common salt). I don't know how high the GH and KH will go with this substrate; I do know the pH will be high, but that is not a problem for livebearers.
You really need to pin down the actual GH and KH (and pH) of your source water. The municipal water authority should be able to tell you these values. Or perhaps a local fish store. I wouldn't waste any money, whatever it is, on a GH test kit unless you do go down the road of adjusting water parameters, and you might need one then, depending how you do it.
I know you have your heart set on livebearers (presumably), but life is much simpler when you select fish suited to your source water. Not only less expensive, but emergency water changes are easy, and the less fiddling you do with water chemistry, the safer for the fish.
Byron.
