3′,6′-Bis(diethylamino)-2-(pyridin-2-ylmethyl)spiro[isoindoline-1,9′-xanthen]-3-one (L) was synthesized for the selective fluorescence and colorimetric recognition of Hg2+ at pH 6.0. In addition, L was useful for imaging Hg2+ in fish kidney and liver tissues using a fluorescence microscope. Spirolactam ring opening of L for Hg2+ recognition is strongly influenced by the nature of the mercury salt and found to be NO3−-induced. Other mercury salts such as HgCl2, Hg(CH3COO)2 and Hg(ClO4)2 failed to induce fluorescence and colorimetric response of L under the same experimental conditions. For instance, the former salt does not exhibit spirolactam ring opening but forms a new ionic compound (H3L)2[Hg6Cl18]·2H2O (1), whose structure has been elucidated by single crystal X-ray diffraction. This might be explained by (1) the higher covalent nature of Hg2+ and, hence, the lower acidity of the metal center and its inability to induce the ring opening reaction, and (2) the bulky anion, in the case of Hg(ClO4)2, which is also ionic, faces steric hindrance to accommodate within the N(Et)2 group upon spirolactam ring opening.