This paper discusses the concept of water quality and stability, describes various indices available to assess the scale formation (protective layer) and corrosivity of water and the methods of their calculation for different tap water resources in Tafila City, south Jordan. The saturation levels are calculated by Langelier saturation index (LSI), Ryznar stability index (RSI) and the calcium carbonate precipitation potential (CCPP). LSI values are negative and range from -0.39 to -1.5 while RSI values range from 8.7 to 9.8. CCPP values are negative and range from -1.77 to -16.76. Although these results indicate corrosion conditions, this image changes upon heating and evaporation of water with the release of CO2. Water samples exhibit high concentrations of Cl-, SO42- and HCO3- as the major anions and Ca2+ and Mg2+ as the major cations. Magnesium has a threshold limit of maximum CaCO3 solubility at about 0.00424 M. Microbiological tests indicate that three of the water samples (the three springs) are polluted with total and faecal coliform bacteria. Moreover, high concentration of nitrate has been detected in one of the springs. Most of the water samples have SO42-/HCO3- ratio of less than 1. At least 94% of the scale composition is CaCO3 (aragonite).