Photographing of fish in aquarium, especially the one not in your home, presents all kinds of problems. First, there is usually not much light, so increasing the ISO is necessary to get sufficiently fast shutter speed. Next, you want to get lens close to the glass to prevent the reflections from showing up on the photo. And last but not least, fish move around quickly, and to get just the right composition can be a challenge by itself. It is very rewarding to browse through the images and see the one like this, with clear main subject and interesting surroundings.