Carbonate Hardness (aka - Alkalinity, Buffering Capacity): Alkalinity or Carbonate Hardness (KH) is a measure of the buffering capacity of water, or its ability to resist change in pH. Aquariums where alkalinity is allowed to drop too long or too low can often suffer a situation where the pH then ???crashes??? to levels below 5.0 - leading to very inefficient or total loss of biological filtration and direct irritation or stress on some fish species. Although commercial chemical buffers are available, natural water relies primarily on a carbonate-bicarbonate buffer system, hence the name hobby use of ???carbonate hardness???. Favorable levels of carbonate hardness are between 3?? and 10?? dH. This kit contains one 10-ml test reagent, a test vial and instructions and recommendations. This kit is for Freshwater (including outdoor ponds), Brackish and Saltwater. Info from United Pet Group