## 4 Discussion
A similar low survival phenomenon using green light was also observed and reported in [43], but in small fish groups (four fish per tank) of adult tilapia under fluorescent lights and other experimental conditions. One possible reason to this phenomenon could be that an aggressive behavior of the fish was observed in a green light environment. Other related study determined (by blood test) the basal plasma cortisol levels (stress response) in adult Nile tilapia, and it was found that cortisol levels increased with green light [44]. This could be a cause of fish mortality because the green light may create stressful environment and increase cortisol levels. A high level of cortisol also affects the nutritional composition of tilapia, causing high moisture levels and low protein levels [45], which agree with our results. The low level of protein can be attributed to an intensive use of muscle protein to get higher energy than that obtained from food, which could be caused by the stress due to green light [46]. Green light may be an important factor of mortality, but research with replicate tanks should be carried out to see all possible mortality factors. [PERSON] et al. [45] reported that hybrid red tilapia in saline water had a lower growth performance under blue light and other experimental conditions. These results were attributed to a reduced vision of the fish under green light, which reduced feeding (also see [47]). In addition, they reported that hybrid red tilapia that were illuminated under red light had the best growth performance [45]. These observations are in agreement with our results, where the Nile tilapia fingerlings in freshwater under blue light had the lowest growth performance, and those under red light had the best growth performance. In addition, in [43], yellow light was shown to have the best growth performance, which also agrees with our results, because the yellow light has a long wavelength that is close to the wavelength of the red light. In our research, the effect of colored light on the nutritional composition of Nile tilapia fingerlings, and on the biofoc composition as a food source for tilapia was also measured, and this has not been reported before. The effect of light on body composition of tilapia showed that the R group had the highest lipid content and also the lowest moisture level (this indicates that moisture and lipid content are inversely related, which is in accordance with [48]). However, under blue light, the fish had the highest percentage of protein and moisture and the lowest level of lipids, while under green light, the fish maintained a low content of lipids and proteins. Under natural light, the fish obtained a high crude protein content and the highest content of ash. The effect of light color on biofocs showed that the G group had the highest lipid content. In addition, the biofocs in the G and Natural groups had the highest protein level. For the ash content, B and Natural groups had the highest percentage. In addition, although the biofoc of B group had a high level of lipids, the fish nutritional composition under B light had a low level of lipids. This opposite relationship could be explained by the stress reduction under blue light [44], and also a stress reduction under the lipid-rich diet [49]. Such a possible stress reduction, would reduce the formation of lipid deposits in fish, indicating that such good health condition of tilapia under B light could be because the lack of stress. The results discussed above may have an important application in tilapia aquaculture with biofoc systems because they improve our understanding of the significant effect that light color has on the growth of fish and their nutritional composition of both the fish body and the biofocs.