Most likely because there is no one setting or range of settings that will universally produce good results. Each photo is dependent on a variety of conditions in an aquarium and in each area of an aquarium. With this type of photography, you can't just put it on automatic and take pictures. There are several sticky threads in this forum that will help you and if you search for my posts in this forum you'll find several instances in which I've offered assistance. If you take a couple of photos and post them, I'm sure I can help you improve the results. It also helps considerably to have a basic understanding of the fundamentals of photography, i.e., the relationship between shutter speed, aperture, and IOS setting, the concept of depth of field, and white balance correction.
Gary