Move the big plants on the left, to the right and try to hide the filter. Change to a sandy subtrate, and get a peice of bogwood, or a really exotic plant, and put it just off centre, but make sure it sticks up in the tank. Your current peice of bogwood is just on the bottom, but if you can get it to go to about half way up the tank, it will look great. All tanks should have a centrepeice I think, unless your going for a symetrical look, but don't make it to centred as this will cause distraction from the rest of the tank. That's IMHO
Neal