Taking pictures of fishes is not an easy task. The best pictures can be taken if you dive into their natural space. But for that you need special equipment and diving skills, unless you happen to be in an underwater aquarium.
Another option is to visit high quality aquarium as in Monterey, CA and take your photos from the other side of a glass.
Keep in mind that there are 3 challenges when taking picture via a glass:
1. You need to avoid reflections of natural light
2. Try not to use flash, or use it carefully
so the reflection will not be seen
take many pictures.
The effect of "Saturation" will emphasize the hot colors:
The adding warmness to the photo will change the look and put into it more fire..red/pink.
Tropical fishes photos needs only small touch and any changes in the
natural colors may only harm the beautifulness of the nature.
and when the flash is reflected, you may get :
