Check out the rock, look for creaveses, notches, and thing you can you to lock them together. Like putting a puzzle together, look to see which pieces fit together.
I would not recommend gluing them together, If down the road you need to take out a piece or move it to maybe get a coral to fit, you will not be able to do so, Just practice and check each piece after you set it on, push and wiggle it a bit. But you will getit.
These are pictures of my 180:
These are some of one of my 75:
These are of my other 75:
Just work with it, and be patient. And you will come up with something great.
None of these tanks have the rock glued, tied, or anything, Just stacked and balanced, and have all the rocks holding each
other together.
Oh forget one very important point, have fun with it. If you start getting fustrated take a break. And you may get fustrated.