I did 2 islands in my 150. I cementedbase rock together to make a dome shaped structure to allow for the best possible water movement around the rock. Also to give fish and inverts a place to hide and swim through. I topped it off with 150lbs of real live rock. It's all sitting on 5" of Special grade/TBS live sand.
I like the crevasse, to peer through, flow is probably very good! Purple looks happy
Nice job.