Hello, I see no problems using cinder block. For support on tank I used 2x4 set up like flooring joists.
The first pic is for a 55gal. I removed center blocks cause it was overkill.
The 2nd pic is a 90gal & the design held perfectly.
On a 7ft tank I would do 3 sets stacked brick. The setup only took a few hrs to make & another hr to paint. Hope this helps!!!!!