The present work presents the experimental study of the adsorptive properties for the removal of Cu2+, Pb2+, Co2+, and Ni2+ metallic ions from water of thermally pretreated fish scales from a very common and abundant source in Mexico, the Oreachromis Niloticus fish, known as "Mojarra Tilapia". To determine the influence and importance that the two main fractions (the organic or protein fraction and the inorganic fraction mainly composed of hydroxyapatite) of this material play in this adsorptive capacity, the original pretreated fish scales are subjected to an acid demineralization and a basic deproteinization treatment to modify the organic/inorganic matter ratios present, and their materials are characterized in terms of their adsorption ability, especially in the removal of the Cu2+ ion. The adsorption isotherms for the thermally pretreated fish scales show that they have a maximum Cu2+ adsorption capacity of 58 mg of Cu2+ Ig of adsorbent with an ion selectivity removal in the order Cu2+ > Pb2+ > Co2+ > Ni2+. When the pure organic and inorganic parts of the fish scales are used in the adsorption experiments, the inorganic part has a 75% higher removal capacity than the organic fraction. The adsorption experiments using fish scales with different organic or inorganic fractions show a nonadditive, synergistic effect on the equilibrium amount of metallic ion adsorbed, which is independent of the inorganic content between 30 and 90 wt % of this inorganic fraction. Low-vacuum scanning electron microscopy and elemental analysis reveal that the ion-exchange reaction is the main mechanism for the metallic ion adsorption by fish scales.