I think my long answer got lost somehow. The plants and pagoda are similar in size. The snail contrasts with the rest of the elements in the tank since it is the only thing of its size. The little snail is keeping the pagoda from being the focal point. Adding some rocks (as suggested) that are the same scale as the snail (in groups, not scattered evenly) would help. If you add a large rock, you should probably add more than one because unique elements either are focal points or compete with the intended focal point. And they probably should not be the same scale as the pagoda—if you want the pagoda to be the focal point, keep it as unique as possible. A search for “artistic composition” returns articles that go into detail about contrast, scale, grouping, repetition, focal point, etc.
Also, as your tank matures and your plants grow, things will get better on their own because the plants will eventually be different sizes than the pagoda and each other.