Marine pH & Alkalinity The Marine pH & Alkalinity test features a unique dye that has a pronounced color change between 8.0 and 8.3, making it the easiest kit on the market to read in that critical range. It is suitable for marine or brackish water only. Alkalinity may be read from the same sample in 0.5 meq/L increments. Ammonia The ammonia test measures total (NH3 and NH4+) and free ammonia (NH3 only) down to less than 0.05 mg/L and is virtually interference free in marine and fresh water. Free ammonia is the toxic form of ammonia and thus it is much more important to keep an eye on the level of free ammonia in your system. This test may be used with freshwater or marine water. Nitrite/Nitrate The Nitrite/Nitrate test measures nitrite to less than 0.1 mg/L and nitrate to less than 0.2 mg/L in marine or freshwater. The values reported are expressed as nitrite and nitrate rather than nitrogen (to convert to nitrogen, simply divide by 3.3 and 4.4, respectively).