I agree that you must find out the GH especially. Before spending money on a test kit you may only use once, can you get the GH and KH from your municipal water authority (assuming you are on city water)? Check their website, many post this in the water data.
I have never bothered with TDS so I don't know what it may be. My GH and KH are zero (tap water is 7 ppm GH) and the pH remains on the acidic side, varying according to the individual tanks' biology. I have wild caught soft water species that are thriving and spawning. I'm not saying TDS is not important, but the GH measures the "hard" minerals primarily calcium, and these block the kidneys of soft water species so this is thee critical area.
Depending upon the GH, you may not want to use any remineralizer. I went through this a few years ago using Seachem's Equilibrium for the plants; this certainly made a difference, but when I learned from a couple biologists how these additives get inside fish and affect them, I quit. There are other safer methods for adding minerals for plants without risking fish.