In article , Chris Spierings wrote:
Greg,

I built a stand for my 90 gallon using 2x4as at each corner and another
one front and back at the mid point. I'm pretty sure the 90 has the same
footprint as a 120 but its just a bit shorter.

I made sure to stack all the wood before fastening. That is make sure
that gravity was working to keep the joints together rather than relying
on just the fasteners to do the work. The back is open. The sides and
top which the tank rests on are 1/2" Oak plywood.

I would also put plywood on the back. This will add rigidity to the
stand keeping it square. This type of frame is very strong, but any
side to side movement and the stand can quickly foldup and
collapse. You don't have to cover the back completely. Atleast the two
corners and the top of bottom horizontal beam should be connected
together with a single sheet of plywood. Although covering all the
back is better. You can put cutouts in the ply wood for ventilation,
plumbing, power cords, etc.

This stand discribed by Chris is very strong. Several times stronger
then it needs to be, but that good. This style stand can easily be
used for larger tanks by adding more vertical pieces to the front and
back. 2x6 are often used for larger tanks as well, although that
probably is not necessary.