Excellent thread. Thanks for the SketchUp tip. I've already downloaded it and am sure it will come in handy.
I'm in the planning stages for a 10x3x4 marine tank. Plywood, very similar to your construction. A couple things I've planned differently. I'll use 2x4's on the top to keep the sides from bowing. I don't want to use metal due to corrosion. I'm really cheating with my lights as I'll just use a roof tube (like a skylight) and have natural sunlight (in 2 locations, about 6' apart). Hawaii is in the tropics, so no worries about temperature or photo period.
The biggest difference is my structural design is the use of what I call a "plywood sandwich". Basically similar to your design, but using 2x4's sandwiched between 1" (or 3/4") plywood. The sheer strength of the plywood is incredible (we have an occasional earthquake here, so need some extra sheer tension).
I haven't priced it out, but I'm thinking it is a lot more economical than using 2x6 lumber (plywood is cheaper here than framing lumber).
Also, I've recently changed the plans to use 2 sheets of glass 36x54. So the front will have 2 viewing panes (instead of continuous view) but it kind of adds to the mystery of the tank (and is a lot cheaper and easier to install). (Just throwing that out there for others following this thread...).
The size tank that you built and your construction practices are no doubt a "best practice" (uses the best principles from other great threads on this site). I'll likely modify my plans after studying yours more. Many thanks for that.
Anyway, great thread and if you have any thoughts on the above plans, please let me know.
Aloha!