The whole point of using 2x8's instead of 2x6's for the horizontal pieces is so that you won't need an additional vertical "center support", but it can't hurt, and it will reduce the <1/8" deflection to almost zero. But don't buy any extra wood for it (like a 2x6) just use a piece of 2x4 or whatever you have laying around.
Using two 2x8's on one side is unnecessary, and complicates the design, a single 2x8 is more than sufficient.
I wouldn't go thinner than 3/4" plywood
If you must double up the 2x8's, I like the design on the right better. The design on the left is making a 2x4 do double duty, as both a leg, and as a screw strip. If the two legs aren't exactly the same length, or the floor under them isn't exactly flat, then there will be sheer stress on the screws. If you use pocket screws you don't need a screw strip, but a screw strip is better because it provides some protection against wracking (folding) that pocket screws do not. If you use pocket screws, you'll need to skin the sides, or at least use triangles in all the corners (I'd do that even if I used screw strips). I suspect that sheet rock (dry wall?) is a sufficient skin. Even a medium sized earthquake can move a couple inches side to side, and without more support those corners could fold with that much weight on top trying to NOT move side to side.
There will only be about 15" between them, so yes, more than good enough. 2x6's would also be more than enough.
For sure you can substitute 2x6's for 2x8's everywhere except maybe for the double's along the back. While a single 2x8 would be fine there, two 2x6's probably wouldn't be. The cross members will be hanging on only the inside one of the pair, so it will be taking much more than half of the load for the pair. Also, even a very slight bowing of the "flat" top will cause the plywood to push down along the inside edge of the inside 2x6, and push down very lightly, if at all, on the outside one. Using two instead of one is a fundamentally different design, and so is unstudied and untested. The inside 2x6 would be holding most, if not all, of the weight for the pair, and we know a single 2x6 wouldn't be enough. Using a single 2x8, on the other hand, we know for sure will work.
Sure, no problem. As long as the end of every horizontal piece has at least part of a vertical leg under it, you'll have no problem.
The catch trough is a really good idea. 1/4" or 3/8" plywood would be fine down there.Planning on using this template for my new 8x2x2 aquarium that ought to be arriving around Thanksgiving week
One thing I've not seen mentioned is putting a base of 0.5" or 0.75" plywood underneath the bottom 2x4's. This ought to be ok, right ? I'm just trying to create a trough to put the sump into, so that if ([insert deity] forbid) it does overflow, the hardwood floors aren't the first target of the saltwater...
The tank appears to require support everywhere, despite being a glass tank - GlassCages insist I should have a plywood top, and then put the 0.75" styrofoam on top of that, and then the tank. I guess they know what their tanks need... You'd think it was acrylic or something...
So, I'll end up having plywood on the top, on the bottom, and at least 1/2 way down the back (the top-half, I need to be able to get to the mains sockets behind the tank when it's done). Is that likely to provide sufficient "skinning" for strength ?
I'm planning on using 2x8 for the top 96" span, with a center brace at 48". From what I've read, that ought to be fine. Just wanted to confirm
Cheers
Simon...
