Imperatorfan had some questions in a PM. I thought I'd answer in the thread just in case anyone else is in a similar situation.
The 180 will have a lot more weight, thats for sure! I really want something that large but we don't have room in the great room for it in this house. We decided that we had to stay with a 4' wide tank.
Here's a useful link to aquarium sizes and weights.
According to that, you're looking at roughly 6' by 2' footprint and over 2,100 pounds to support.
I was able to find data on the mechanical properties of kiln dried red oak, but I don't have any comparison to plywood. I do know from experience that plywood will not support as much weight. How much of a difference is it? I don't know on something this large. I know from experience on 45 gallon and under that plywood is plenty sturdy. That may or may not mean that it would work for a 180.
What I do know is that Oceanic and All-Glass make stands for 180 gallon aquariums. Take a look at the way they are constructed and use that to guide your design. I'll bet they are made out of 3/4" material and they hold up just fine.
The reason that they do is because they are commercially constructed and are perfectly square (and plumb). Look at the way that they are braced internally to prevent the long 6' sides from bowing out. Look at how the corners are constructed. I think you mean to double up the 3/4" plywood. If you double the material up, make sure that you get an even coating of wood glue to bond the inner and outer sheet together. In my opinion, that would be a good alternative to 4x4 construction as long as the stand is well constructed.
Some DIYers can make near perfect cuts and some can't. You know your capabilites and what tools you have available. Use that to help guide your design decision. If there's any doubt, go for heavier construction.
Now, how would I tackle this? Frankly, I would be worried about that size of an aquarium on 3/4" material. The idea of doubling up on 3/4" material seems to be a real winner to me. I think you'll end up with a sturdy and professional looking stand. It is going to be more difficult to pull off because the jointery is critical. On a Garf stand, it isn't as critical (within reason).
The modified Garf design is proven, so I can't not
recommend it. I really do not like pine 2x4s and 4x4s because they have a tendency to warp, cup, and twist. However, on something this size, I would be inclined to overlook that shortcoming. Notching the 4x4 uprights so the top frame of 2x4s will interlock is very easy to do, but time consuming. You can do it with a circular saw and a chisel. Those are both very common tools.
I would then line the inside of the cabinet in plywood (birch or oak veneer if you like) to hide the pine framing members. Of course you should determine your sump requirements, if any, and ensure that the interior will accomodate.
Whatever you decide, please share your thoughts with us. Start a building log so we can all see new and creative solutions to this common DIY