Product Description: What do your fish swim in? A TDS meter (aka ppm pen, nutrient tester, salt tester) is an inexpensive and convenient digital tool to instantly check your overall water quality. TDS = Total Dissolved Solids, which is any salt, metal or mineral in the water. The TDS-4TMA uses the NaCl conversion factor, so it can also be used for testing salt levels. With the push of a button, the TDS-4TMA can tell you your overall water purity level, which will be displayed on the screen in ppm (parts per million). 100 ppm = 0.01%. The lower the TDS level, the purer the water, with 0 ppm being pure H2O. The TDS-4TMA is great for checking your freshwater aquarium, koi pond or water filtration/purification system, such as reverse osmosis (RO) or deionization (DI). (The meter does not test for biological contaminants.)