I'm still worried about those connections at the top. Each of those points are pretty highly stressed, although I don't know the physics that others can factor for you.
Basically all the pressure along the linear length is on those three stress points where the cross pieces are glued and screwed. All the pressure is on something like 1.5" square inches at six specific locations, and up high where you don't have much material reinforcing the area. I can see how the tank is a tad wavy in one of your images.
If you were to run a strip of that half inch stock along the outer length (or the inner one for that matter) and glue it in place, you'll be helping that area's rigidity and the cross brace would reinforce it's larger surface area.
Think about this for a second. It is one thing to fill it up with water and watch for failure. It is another matter with day to day activities, leaning against it, reaching in and out, having LR topple over against it, someone bumping into it, or god forbid something fall against or on it. If you plan to make 20 or 21, they should be something reliable for a long time. Sure, things will happen and one will have to be repaired for some reason, but these really are too flimsy in my opinion. I had to build an 80g sump with 1/4" acrylic once, and it was tough maintaining shape and integrity. You have almost 3 times the volume, and it is 7' long. That is a lot of pressure in the center of each long panel.
Even a welded collar that goes around the tank and rests on the top would be ideal. It could be like angle iron, but only be 1" down, and 1/2" across (if the material was 1/4" steel). There would be no lip hanging in the tank, but the collar would give the unit more strength along the top, where you need it most. It would look like this:
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