I like the rockwork you've done on the right side of the tank but the rocks you've placed on the left give the tank a much too symmetrical look (although I have to say that you've got some very nice looking rock for your 'scape). Their shape and placement looks like a mirror image of those on the right and appears a little unnatural to me. Unbalancing them by breaking up the left group, possibly moving some of them a bit forward and playing around with their angles, would help give your tank a more natural look.
In a small tank like this one, with rocks as proportionally large as yours, you should consider using only three of the larger rocks, possibly adding some of the smaller pieces in the foreground and to the sides. Iwagumi, the Japanese art of stone arrangement that Takashi Amano has so brilliantly adapted to aquascaping, uses odd numbered groupings of prominent rocks to create focal points and to produce a natural, asymmetric feel. The most popular style, sanzon iwagumi, uses three main rocks in its arrangements: one large rock complemented by two somewhat smaller rocks that are placed at varying distances from the main rock, often leaning reverentially toward it. Viewed from the top-down, the three primary rocks in a sanzon iwagumi arrangement may often be seen to form the points of a scalene triangle (one having three unequal sides). The relative positions and angles at which these three rocks are placed, along with their shape and texture are key factors in determining where the focus of the arrangement will be.
The focal point of the iwagumi design (which can be the rocks themselves or a spot toward which the rock arrangement leads the eye), tends to look most natural when it is not dead-center. In the case of your last picture the two similar groupings of rock on either side of the tank, although each looks great on its own, focus the eye on the open area at the very center of the tank and give the 'scape (to my eye anyway) an unnaturally balanced look.
An off-balance iwagumi arrangement with the focal point roughly 2/5ths of the way in from either the left or right side will help lend your tank a more natural appearance. I'm not sure why 2/5ths works so well (try searching for "golden ratio" here on APC to read more about it) but I have to admit that it really does seem more organic. Take a look at some of Amano's iwagumi-style tanks (or for that matter some of the amazing iwagumi tanks from members here at APC) and they will give you a much better illustration of what I've poorly tried to explain here in four rambling paragraphs.
