Tetratest KH Test Kit

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.