For the adsorption development of a dissolved ammonium removal method in RAS, chitosan-based hydrogel (Cht-HG) modified with 0.3 mM of p-hydroxybenzoic acid (Cht-0.3PHBA-HG) was devised. An additional function of ammonium detection under Ultraviolet (UV) displaying fluorescence depends on ammonium concentration. Between control hydrogel, Cht-HG and Cht-0.3PHBA-HG were compared in the ammonium adsorption test, and the maximum adsorption capacities (qm) of Cht-HG and Cht-0.3PHBA-HG were 11.6 mg g-1 and 19.9 mg g-1. Cht-0.3PHBA-HG exhibited a relatively elevated adsorption capacity (i.e., 1.71-fold higher adsorption efficiency) due to the morphological and chemical modifications attributed to p-hydroxybenzoic acid (PHBA). The influence of the adsorbed ammonium into Cht-0.3PHBA-HG was investigated under UV light and exhibited a steady fluorescence quenching at 396.5 nm with the addition of increasingly higher ammonium concentrations. Therefore, Cht-0.3PHBA-HG could be used as an ammonium sensor. Furthermore, the correlation coefficient (R2) was 0.951 for ammonium determination ranging from 0.1 to 5.0 mM, and the limit of detection (LOD) was 0.1 mM. For the practical application, Cht-HG and Cht-0.3PHBA-HG were assessed with an actual effluent including 2.24 mg L-1 ammonium concentration of a fish farm and the ammonium adsorption ratio were 33.0% and 60.1%, respectively. These results explain the modified hydrogel has the improved practical adsorption efficiency and sensing ability for ammonium.