What chemical should be use to reduse the ammonia concentration in water?

Ammonia is a weak base. If you want to reduce it, you need a weak acid. What weak acid can you think of?

Post your answer (suggestion) :)

Actually any acid will do since ammonia in water is in equilibrium with ammonium ion and hydroxide:

NH_3 + H_2O \leftarrow\rightarrow NH_4^+ + OH^-

Adding any acid will reduce the concentration of hydroxide this will cause a shift in the equilibrium position towards the products, reducing the concentration of ammonia in the solution.