I like what you've got with the 2nd shot. Somehow the rock formation is a bit off centre to me and gives the tank a shorter appearance lengthwise. You could taper the right side by skimming off some rocks and elongate it. Maybe do what Coler said about running another smaller rock stack from the right tapering to the left behind or in front of the first stack. You've already created this illusion of depth by rounding the rocks on the left around the front corner.