I like that look a lot - have you left space at the back and sides between rock and tank ?
I'd go a lil higher, and if you want this look :
1. take out a few from the end - shorten the line.
2. run a much lower/smaller line from back right hand corner (the empty back corner) diagonally right to left towards the front of tank, so you have it sort of overlapping the main line.
But its good as is - nice job