To me your going backwards, find your main centerpiece plant and then plant around it. I would take those helferi on right and cluster them all around rock on left. Then right in middle of where they were Iíd put a dwarf water lily with big, bold, wide red leaves. Then move a couple of your erectus so they circle around to left of lily bulb and come forward a bit on left.
All your leaves are small, layout is to symmetrical. You need a slightly lower center piece splash that may require a wide planting swath and actual limit how many plants you can fit behind it. This is a good thing, it creates a field of depth where you can see farther back into tank. The taller plants will fill in on left and right, that way itís not just a solid wall of plants from one end to the other.