Nitrates come from dissolved protein, such as fish’s excreta, residual food, and dead fish. The compound of nitrogen exists in forms of NH₄ and NH₃. NH₃ is toxic,but NH₄ is non-toxic and can be absorbed by water plants. NH₄⁺and NH₃ will fluctuate with ph value . NH₄+level will be higher in acidic water and NH₃ level will be higher in alkaline water . In fresh and salt water aquariums where ph value is above 7.5, NH4/NH3 test become a very important work.