Testing for General Hardness (GH) should be done to determine if the levels of Calcium (Ca) & Magnesium (Mg) are optimal for specific species of fish. Carbonate Hardness (KH) levels need to be tested on a regular basis because they fluctuate over time & can negatively impact the p H balance of the aquarium water. Low Carbonate Hardness will result in poor plant growth. Suitable for fresh & salt water.