Marco,
There's something uneven with the lighting. Seems to be all in the centre making it very gloomy at the sides. Was it you who mentioned on another thread that you wanted to change the bulb colour as well. That would be a good idea. It's too yellow. Almost looks like an incandescent bulb. Are you in Australia? Edit your profile to indicate that so we don't give you inappropriate advice for your location.
If it were me? I'd ditch that background because it makes everything look claustrophobic - but that might be due to the lighting. It would be much more interesting with a moss wall, for example.
You also have a very uneven distribution of heights and textures, which makes it more random than thematic. Have a read of George F's Aquascaping 3 part article sticky and think about how you can apply the principles. For example you can move some or all of the taller plants to one side and taper to lower heights towards the front or towards the opposite end. You could do with a bit of trimming on many of those stems to get a more manicured look.
Our brains actually respond to series of lines and geometric forms, so random shapes and heights "look" disruptive. Think first therefore in terms of an arrangement of shapes such as two triangles side by side, or a single square in the middle, or three progressively smaller triangles - whatever combination - then fit the plants into those shapes. Look at some of Georges or other peoples compositions and see if you can guess the themes of the broad shapes. Then see how you can implement with those plants that you have or those you can acquire.
Cheers,
