Rockler I-Sembles for tank stands
 

Has anyone seen these I•Semble shelf blocks yet?
http://www.rockler.com/product.cfm?page=30930

I haven't been able to find out how much weight they could support. (I assume it would vary based on the shelf materials.)

If appropriate, they might be useful for easily creating attractive tank stands or racks

Those would hold the weight of a tank.
The problem is they are only 13 inches tall. That will be limiting given the design of them.

18G Steel...looks pretty strong except for lateral forces.