A few thoughts:
I am not an engineer, so take what I have to say with a grain of salt. I know that "moment" is force over distance. It is relevant here in that a longer piece of material will bend more than a shorter piece. How much it will bend for a given length when subjected to a given force is measured by an object's "moment of inertia", which is listed for the products on the website.
Moment of Inertia,
https://en.wikipedia.org/wiki/Moment_of_inertia
It can be calculated what thickness of material would be needed to support a given weight. This is beyond my abilities. Junction points will probably lessen the strength of the structure, and adding height to the structure would make it unsteady at some point. It's interesting that the moment of inertia for the thickest gauge rod isn't too much more than for a run-of-the-mill pine 2x4:
https://www.engineeringtoolbox.com/structural-lumber-properties-d_1835.html
So, if I were doing this, I would build to the same specs as if I were using 2x4s, assuming the corner pieces don't weaken things. I'm sure 2x4 stand plans for your size aquarium exist somewhere. If they don't, and you have to wing it, you'll want to make the structure considerably stronger than it would have to be just holding the tank under ideal circumstances. I don't know what you'd have to do to protect the aluminum from salt damage. The best thing (not the easiest) would probably be to clean it really well, dry thoroughly, and immediately spray (with an air brush) a thin layer of Gripper primer, followed by paint. Or, you could put a thin layer of auto wax on it, and hope for the best.
As for making a stand out of wood, it's actually not that difficult. You will need to cut or have cut wood to
precise lengths. Also, you will need bracing clamps, which cost about $12 each (I think). I doubt you'd need more than eight of them, and you can resell them when you're done. There's a bit of a trick to using braces, so you can PM me if you go this route. Trying to build a squared box without clamps is futile.
As for finishing, personally, I'd use pure tung oil, not some stuff called tung oil which is actually linseed oil, but real tung oil. This stuff has some great features, one of which is being easy to repair. The two downsides are the cost and the drying time (about seven weeks).
The only advantage that I see to the aluminum frame over wood is its light weight, in case you ever want to move it. Considering how ridiculously expensive the aluminum is, you could have a carpenter make a high quality stand with screws, dovetails, or "post and beam" construction for as much as you'd pay for just the aluminum raw material. If you go this route, don't use maple, as it soaks up water like a sponge.
