Looks to me like most of the plants are artificial with just a few live plants. Natural looking tanks are great, but I think fake decor and plants can also be layed out in ways that are very beautiful too. From a design standpoint, it doesn't matter whether you're working with a big piece of driftwood/rock and live plants or a big fish tank decor item with artificial plants. For the best presentation, follow the same kind of composition techniques that nature photographers would use when cropping a photo.
Looking at this photo, I would say it looks "messy" because everything is arranged symmetrically. You have a large decor item sitting right in the middle with practically a mirror-image arrangement of plants on both sides. Using the exact same material, a better layout would be to move that castle over to the left or right side. Then "balance" the visual accent and heaviness it has by grouping taller plants on the other side. Low and heavy on one side, with tall and stately on the other. Everything else kind of falls into place after that.