Three species of corals at Sichang Island i.e. Acropora sp. Goniopora sp. and Pavona sp. were subjected to a stress test with low salinity and normal salinity at concentration 10, 20 and 30 psu, respectively. Under water photograph and eye observation of coral activity were recorded at 12, 24 and 48 hours. The entropy or surface roughness and percent polyp activity were analyzed with comparison to eye observation of coral activity. The experiment was carried out under continuously water temperature and underwater light intensity controlled. The results indicated that “Healthy” entropy values for Acropora sp. are 1.57–1.62 and for Goniopora sp. are 4.26–4.46. In contrast, for Pavona sp., short polyp coral, there was no “Healthy” entropy value resulted from any photographic assessment in this study. The “Healthy” value of Acropora sp. evaluated from percent active polyp was more than 52.4. For conclusion, The Entropy and percent active polyp values were suitable tools for Acropora sp. and Pavona sp., a short polyp coral's healthy identification. Whereas the eye observation of coral activity both percent polyp extend and percent polyp long was suitable for Goniopora sp., a long polyp coral.