Have a window in the same room as your aquarium? The additional light can cause algae issues and throw off the internal clocks of your livestock. The fish will be active even if the lights are turned off and other photosynthetic organisms (like Tridacna clams) might try to orient themselves to the source of light. This may not be much of an issue to most aquariums and their inhabitants, but for some it can present serious problems, especially if they’re already having trouble with dinoflagellates or cyanobacteria. A simple set of curtains can prevent the extra light from coming in, and are recommended as blinds may not keep the light out completely.