In addition to this, if the 2x4 was only touching on the top OR the bottom, the screws securing the 2x4 to the rest of the frame will be taking the entire load of the tank. Screws are very weak in this direction (shear force) and they are very likely to break. The way the stand is built, the only shear force that the screws have to withstand is the weight of the 2x4 itself.
So you're saying that the 2x4's inbetween the top and bottom frames aren't transfering the load of the tank from the top frame to the bottom frame to the floor?
I don't see any flaws in the design at all, the top frame is supported by the bottom frame to the floor via the corner 2x4's. But I still don't see why the floater cannot make contact with the floor at all.
Regardless of whether they touch the floor or not, the brunt of the load is distributed through the corners.
I don't disagree that the design is a good one, I just don't understand/agree that the "floaters" cannot touch the floor. I do agree that the "floaters" are there to keep the corners secure. In the original design the OP didn't state the use of ply on the top of the stand.
So back to my original statement, since you skinned the top with ply, that will distribute the weight as well as
the "floaters" had they made it the full length and made it to the floor.
Additionally, if you were to skin the sides of this stand with 3/4 wood it would essentially be doing the same thing as the "floater" had it gone to the floor.
Majority of stands that are sold are nothing but 3/4" ply slapped together and they hold the weight of the tanks just fine. I personally don't like those stands as I tend to overbuild things myself.
I basically built mine the same way but didn't have the priveledge of finding a thread to give me cut sizes but it looks like a well built stand. I did the corners similarlly but my floater goes to the ground and there is also 3/4" pine that wraps the 2x4 frame. I also put on 3/4" ply on top over all of the main structure, and then put trim around all of that. I also added an additional 1/2" foam on top for extra piece of mind. This was for a 215g Oceanic with 3/4 glass, 3-400#'s sand, 100+#'s holey rock and water.
So that said, I think you did a nice job BS!!!