stands for Total Dissolved Solids. The meters generally read in microsiemans, with the TDS
being higher with higher numbers. The meters measure the conductivity of the water, so the more dissolved mineral content, the higher the reading. if you are getting a reading of 18 microsiemans, your water must be quite soft, Our tapwater here runs in the 300 to 400 range. A TDs
meter won't measure the quality of the water, but in general terms, if the water is safe to drink, it is generally usable for most fish.