Particular aspects provide novel devices and methods for accurately measuring total ammonia (NH3 plus NH4+) in a solution (e.g., freshwater and saltwater) by spatially proximate, simultaneous and continuous quantitative measurement of solution pH and ammonia. The devices overcome prior art inaccuracies relating to non-homogeneous sampling, and to spatial, temporal and thermal sampling discontinuities. Particular embodiments provide a combination pH and ammonia measuring device, comprising: a submersible member; a submersible non-bleeding ammonia-sensing portion attached to the submersible member and suitable to provide for a continuous visual indicator of solution ammonia concentration; a submersible non-bleeding pH-sensing portion attached to the submersible member and suitable to provide for a continuous visual indicator of solution pH; visual ammonia and pH indicator comparison means suitable for comparative quantitative determination of solution ammonia concentration and pH. Additional aspects provide novel ultra-sensitive devices and methods using same for measuring ammonia in air or solution.