You could use a slower shutter speed. But this is of limited value when it comes to photographing fish, which brings us to the topic of subject movement. A photograph includes a background as well as the main subject. Watch backgrounds carefully and compose pictures for the entire frame. Shoot vertical compositions as well as horizontal.
Some species are better suited to vertical proportions.
Most fish are active and seldom stop moving, although this varies depending on the species. The trick here is to move the fish to a smaller aquarium where there is less room for movement. The problem with this solution is that the fish is no longer in the display tank that you so masterfully aquascaped. To make things even easier, a separate piece of glass as large as the front glass, but small enough to fit in the tank, is used to create a narrow space between it and the front of the tank. As the piece of glass is moved toward the front glass, the movement of the fish is increasingly limited, making it much easier to keep the entire length of the fish in focus.