If your plant are new, they'll thicken up. The back stem plants keep them together, when they get too tall cut them down some and stick the newly cut pieces next to or in front of the stem plants of the same type. Cut every other tall plant when they grow too tall and that will create another effect. If that is hornwort back there in the right side of your tank, just keep it floating and it will grow like crazy. Patience is key when creating a new planted tank. Success is measured in the long term.